Download acceleration, also known as multipart download, is a term for the method employed by software such as download managers to download a single file by splitting it in segments and using several simultaneous connections to download these segments from a single server.
The reason for doing so is to circumvent server side limitations of bandwidth per connection. Because in normal networking situations all individual connections are treated equally, rather than actual file transfers, multiple connections yields an advantage on saturated links over simple connections, both in terms of total bandwidth allocation and resilience. Many servers, however, implement a maximum number of simultaneous connections per client in order to mitigate this.
This is not to be confused with segmented downloading, which allows a client to download segments of a file simultaneously from multiple servers.
However Segmented Downloading is defined as:
Segmented file-transfer (also known as multisource file-transfer or swarming file-transfer) is the coordinated transmission of a computer file sourced from multiple servers to a single destination. It can be applied as well when downloading the same file from the same server in various parts, like some download managers do. A computer program downloads (retrieves) different portions of the file from various sources simultaneously, and assembles the file on the destination computer data storage device.
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.
Large files can be made available efficiently to many other users by someone who does not have large upload bandwidth.
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.