Multilink PPP: One Big Virtual WAN Pipe
MLPPP delivers some key functions to help network managers build multi-protocol enterprise networks. Here are some of MLPPP's strongest selling points:
Mix-and-match WANs: With MLPPP, net managers can configure multiple physical links and virtual circuits as one logical pipe, using different WAN service types (ISDN, frame relay, X.25 and ATM) in the process.
Bandwidth on demand: Circuits can be activated and added automatically to a logical pipe when more bandwidth is needed, or if one of the links in the logical pipe fails.
Big protocol tent: MLPPP handles routing for all major network- and transport-layer protocols, including IP, IPX, Netbios, DECnet and SNA.
Network negotiation: MLPPP has inherited PPP's ability to negotiate configuration options between communicating devices. This means two end systems can set the terms of transmission without requiring manual intervention.
No traffic, no link: MLPPP uses LAN protocol spoofing to keep unnecessary network traffic, such as session keep-alive packets, from traversing the WAN.
The two basic types of inverse multiplexing are circuit-based and packet-based. Under the circuit-based scheme, a data stream is sliced into equal portions, regardless of its contents, with each portion transmitted over a different available circuit. With circuit-based inverse muxing, synchronization of traffic streams is generally handled by hardware.
Packet-based inverse multiplexing is a software-based process. In this scheme, packets are distributed among available circuits according to rules or policies that govern allocation of traffic across circuits. These rules can include segregation according to protocol or percentage-based prioritization. The multilink point-to-point protocol (MLPPP) uses packet-based inverse multiplexing.
One key difference between the two approaches is that packet-based inverse multiplexing schemes can handle multiple circuit types, while circuit-based schemes cannot. With MLPPP, for example, a synchronous 56Kbps X.25 link can be bundled together with a 64Kbps ISDN B channel to create a single logical pipe offering 120Kbps of bandwidth. This bundling ability extends to the full range of available WAN services, including dial-up analog lines, switched 56Kbps services, frame-relay connections and T1 or T3 services.
Another difference is that packet-based inverse multiplexing is seamless to the destination router or end system. In the case of MLPPP, the flow of data appears consistent by means of software synchronization of MLPPP fragments. In contrast, circuit-based inverse muxing requires the receiving device to stop data flow until all bonded channels are characterized for latency. This is the only means by which the hardware-based solution can ensure data is received in the correct order.
Except for applications that require constant bit rate transmission or have strict requirements on circuit latency, software-based solutions are generally regarded as superior because they add considerable value and flexibility to the basic inverse muxing function. In the case of MLPPP, users are able to combine different WAN connections while taking advantage of PPP's configuration negotiation and multi-protocol routing support.
This article was originally published by Data Communications and can be found on the web at http://www.data.com/tutorials/multilink_ppp.html.
George E. Conant is a cofounder of Xyplex Inc. (Littleton, Mass.), a maker of internetworking products.
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