MPOA: Multi-Protocol Over ATM

The Multi Protocol Over ATM (MPOA) deals with the efficient transfer of inter-subnet unicast data in a LANE environment. MPOA integrates LANE and NHRP to preserve the benefits of LAN Emulation, while allowing inter-subnet, internetwork layer protocol communication over ATM VCCs without requiring routers in the data path. MPOA provides a framework for effectively synthesizing bridging and routing with ATM in an environment of diverse protocols, network technologies, and IEEE 802.1 virtual LANs. MPOA is capable of using both routing and bridging information to locate the optimal exit from the ATM cloud; it allows the physical separation of internetwork layer route calculation and forwarding, a technique known as virtual routing.

Based on ATM UNI signaling, LAN Emulation, and Next Hop Resolution Protocol (NHRP), MPOA defines two components: MPOA Clients (MPCs) and MPOA Servers (MPSs), and the protocols that are required to communicate and receive services.

The MPS is a component of a router, and is only useful in a router that has an Next Hop Server (NHS) and interfaces to one or more LAN Emulation Clients (LECs). The data and control path from the router through the LEC(s) to LANE is unaltered by MPOA. The MPS does, however, interact with the router, its LEC(s), the NHS, and other MPOA components. A LEC is associated with a single MPS.
MPOA uses a protocol based on the Next Hop Resolution Protocol [NHRP] to manage caches and establish shortcuts. It performs the following operations:

• Configuration- Obtaining the appropriate configuration information.
• Discovery- MPCs and MPSs learning of each others’ existence.
• Target Resolution - Determining the mapping of a Target to an egress ATM address, an optional Tag, and a set of parameters used to set up a Shortcut Virtual Channel Connection ( VCC) to forward packets across subnet boundaries.

• Connection Management - VCCs Creating, maintaining, and terminating for the purpose of transferring control information and data.
• Data Transfer - Forwarding internetwork layer data across a Shortcut. MPOA components must support the use of Logic Link Control (LLC/SNAP) encapsulation for all PDUs. By default VCCs must be signaled to use LLC encapsulation. An MPOA component must be capable of establishing, receiving and maintaining a VCC to any entity that conforms to the connection management procedures, whether or not that entity is an MPOA component

MPOA tagged encapsulation format:

Three bytes of Logic Link Control (LLC) header with proper value as indicated must be used and three bytes of SNAP header with the Organizational Unique Identifier (OUI) must be used, which should be filled with “0s” in this case.

Again, three bytes of Logic Link Control header with proper value as indicated must be used and three bytes of SNAP header with the Organizational Unique Identifier (OUI) with proper value as indicated must be used.

By default, MPOA uses LLC encapsulation for all control flows as defined in [NHRP], with the same fixed header as an NHRP packet described below:

• ar$shtl - The type and length of the source NBMA address interpreted.
• ar$sstl - The type and length of the source NBMA subaddress interpreted.

MPOA is jointly defined by ITU-T and IETF.