Multicast fan-out saves bandwidth

Multicast technology is the most efficient way to deliver a file or data stream from a single server to multiple clients. The use of multicast conserves server resources while reducing bandwidth requirements, which can be especially beneficial over expensive WAN links.

While there is little dispute over the benefits of multicast, it is not widely deployed because of two particular limitations.

Graphic: How multicast fan-out works

First, complex billing, management and security concerns have caused most ISPs to block multicast traffic from their networks. Because it is necessary for the entire network path between the server and clients to be multicast-enabled, multicast availability generally is limited to closed networks.

Second, IP Multicast, a standard component of the TCP/IP protocol suite, is a User Datagram Protocol (UDP)-based, unreliable, "best-effort" service. While useful for real-time streaming, IP Multicast is inappropriate for file and data transfers where reliability is a requirement. Although there are a variety of products available for reliable multicast, these generally require the installation of multicast protocols on all end nodes and can use only specialized multicast-aware applications.

Multicast fan-out

Multicast fan-out (MFO) overcomes these limitations by transparently converting unicast TCP connections into a specialized reliable multicast protocol for transmission over the WAN. On the opposite side of the WAN link, the process is reversed - the reliable multicast protocol traffic is converted back to unicast TCP for communication with the ultimate destinations.

This fan-out process lets the server and each of the clients continue using standard TCP-based applications, including FTP, without any changes to the local network infrastructure, while taking advantage of the benefits of reliable multicast over the WAN.

MFO works by inserting an MFO transmitter on the server side of the WAN link and MFO receiver at each remote site. This functionality can be implemented as a separate proxy-like hardware device or embedded into other hardware on the network such as the router, modem or WAN connectivity device.

The MFO transmitter intercepts the unicast TCP connection from the application server. The MFO Transmitter then sends the data over the WAN link to each MFO receiver using a specialized reliable multicast protocol. At each remote site, the MFO receiver relays the data to a local client or set of clients using a new unicast TCP connection (see graphic).

MFO can work with any TCP application that pushes data from a content server to local clients. Uses can include not only file transfer applications such as FTP, but databases, cache replication systems, content-delivery networks, video servers and proprietary client-server applications.

Time and bandwidth savings

Over a 2M-bit/sec link, a standard FTP download requires 400 seconds to transfer a 100M byte file to one site. To transfer the same file to 10 sites requires more than an hour. With 100 sites, the transfer takes more than 11 hours.

Using MFO, the file transfer takes 400 seconds whether sending to one, five, 10 or 100 or more remote sites. Similarly, MFO uses no more bandwidth to send the file to all the sites as it does to send it to a single site.

MFO is ideal for distributing content stored at a central location to multiple remote sites. With MFO:

ISPs can replicate cache content to local points of presence.

Chain stores and franchises can download database, inventory and pricing data from headquarters to local stores.

Companies can distribute software updates, presentations and training materials across the company.

The entertainment industry can transfer large video files to distribution centers, affiliates or directly to theaters.

MFO is transparent to clients and servers and requires a multicast-enabled infrastructure only over the WAN link. By automatically converting standard unicast transfers to reliable multicast, MFO reduces the time to transfer data and files to multiple users while cutting bandwidth costs.

Multicast fan-out saves bandwidth

Multicast fan-out

Time and bandwidth savings

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