Difference between revisions 104912288 and 104912289 on dewiki

{{uUnreferenced|date=October 2008}}
'''Segmented downloading''' (also known as multisource downloading, swarming download) can be a more efficient way of [[downloading]] [[Computer file|file]]s from many peers at once.  The one single file is downloaded, in parallel, from several distinct sources or uploaders of the file.  This can help a group of users with [[asymmetric]] connections, such as [[ADSL]] to provide a high total [[Bandwidth (computing)|bandwidth]] to one downloader, and to handle peaks in download demand.

== History ==
Segmented downloads probably have an origin with [[NASA]] and the [[magnetic tape]] based file systems used on [[Deep Space Network]] craft such as those in the [[Voyager Program]].  

NASA missions using some kind of segmented downloading
* [[Mars Rovers]] (for [[ICER]] image files)
* [[New Horizons]] (for [[Jupiter]] flyby data)
* [[Voyager Program]] (historical)

[[Swarmcast]] was the first significant [[peer-to-peer]] (P2P) content delivery system that implemented a kind of segmented downloading technology. The program and protocol was invented and developed in 1999 by Justin Chapweske and sold to [[Opencola]], which released the software under a [[GPL license]].

A lot of the terms used in segmented downloading technology have their origin with [[Swarmcast]], with [[BitTorrent (protocol)|BitTorrent]] being the only other significant contributor to the terms in use.{{fFact|date=May 2009}}

== Network implications ==
[[Image:Torrentcomp small.gif|frame|In this animation, the coloured bars beneath all of the clients represent individual pieces of the file. After the initial pieces transfer from the seed, the pieces are individually transferred from client to client. The original seeder only needs to send out one copy of the file for all the clients to receive a copy.]]

Most IP networks are designed for users to download more than they upload, usually with an expected (Download:Upload) ratio of 3:1 or more.

Segmented downloading when used by only 20% of an ISP's user base can upset the ISP's network to a point of requiring substantial reprogramming of routers and a rethink of network design.
* Traditional ''web object'' caching technology (like the [[Squid proxy]]) is of no use here.
* Universal adoption of [[IPv6]] cannot help either, as it only allows all users to have fixed IP addresses. Fixed IP address don't fully address the routing table problems associated with segmented downloading.
* Typical downloading configurations can have a single user in touch with up to ''10 to 30 ephemeral users per file'' scattered across the global internet.
* IP router tables can become bloated with routes to these ''ephemeral users'' slowing down table lookups.

== Network advantages ==
Segmented downloading networks do have some advantages
* routes to the more obscure parts of the Internet can assert themselves across most of the Internet -- —this is especially true for dial-up users
* segmented downloading does save some transmission capacity, as the number of lost or redundant megabytes is minimal compared to losing a prolonged [[http]] or [[ftp]] download

Most ISPs have learned to cope with segmented downloading technology, but coping has meant the mandatory deployment of TCP/IP traffic shaping technology.

== Limitations ==
(contracted; show full)ime. Older clients could only resume a file from the same source. This feature is different from segmented downloading, due to the ability to connect to multiple sources at the same time.  However, new protocol extensions as well as the alternative ADC protocol have turned segmented downloading into an implementation issue instead of a protocol one (and is used in some clients such as the aforementioned RevConnect).

== See also ==
* [[Peercasting]]

[[Category:Data transmission]]
[[Category:File sharing]]