network protocols handbook.pdf

162Protocols GuideWAN - ATM ProtocolsProtocol NameMPOA: Multi-Protocol OverATMProtocol DescriptionThe Multi Protocol Over ATM (MPOA) deals with the efficienttransfer of inter-subnet unicast data in a LANE environment.MPOA integrates LANE and NHRP to preserve the benefits ofLAN Emulation, while allowing inter-subnet, internetwork layerprotocol communication over ATM VCCs without requiring routersin the data path. MPOA provides a framework for effectivelysynthesizing bridging and routing with ATM in an environment ofdiverse protocols, network technologies, and IEEE 802.1 virtualLANs. MPOA is capable of using both routing and bridging informationto locate the optimal exit from the ATM cloud. It allowsthe physical separation of internetwork layer route calculationand forwarding, a technique known as virtual routing.Based on ATM UNI signaling, LAN Emulation, and Next HopResolution Protocol (NHRP), MPOA defines two components:MPOA Clients (MPCs) and MPOA Servers (MPSs), and the protocolsthat are required to communicate and receive services.The MPS is a component of a router, and is only useful in a routerthat has a Next Hop Server (NHS) and interfaces to one ormore LECs. The data and control path from the router throughthe LEC(s) to LANE is unaltered by MPOA. The MPS does,however, interact with the router, its LEC(s), the NHS, and otherMPOA 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 performsthe following operations:• Configuration- Obtaining the appropriate configurationinformation.• Discovery- MPCs and MPSs learning of each others’ existence.• Target Resolution - Determining the mapping of a Targetto an egress ATM address, an optional Tag, and a set ofparameters used to set up a Shortcut VCC to forwardpackets across subnet boundaries.• Connection Management - VCCs creating, maintaining,and terminating for the purpose of transferring controlinformation and data.• Data Transfer - Forwarding internetwork layer dataacross a Shortcut.MPOA components must support the use of LLC/SNAP encapsulationfor all PDUs. By default VCCs must be signaled to useLLC encapsulation. An MPOA component must be capable ofestablishing, receiving and maintaining a VCC to any entity thatconforms to the connection management procedures, whetheror not that entity is an MPOA componentProtocol StructureMPOA tagged encapsulation format:0 LLC- X“AA” LLC- X“AA” LLC X“03” OUI-X“00”4 OUI-X“00” OUI-X“00” Frame-Type = 0x884C8 MPOA Tag12-n Internetwork Layer PDU (up to 2^16 - 13 octets)MPOA Control Frame – MPOA tagged encapsulation format:0 LLC- X“AA” LLC- X“AA” LLC X“03” OUI-X“00”4 OUI-X“00” OUI-X“5E” Frame-Type = 0x0003MPOA PDU8-n(up to 2^16 - 9 octets)By default, MPOA uses LLC encapsulation for all control flowsas defined in [NHRP], with the same fixed header as an NHRPpacket described below:0 ar$afn ar$pro.type4 ar$pro.snap8 ar$pro.snap ar$hopcnt ar$pkstz12 ar$chksum ar$extoff16 ar$op.version ar$op.type ar$shtl ar$sstl• ar$afn - Defines the type of “link layer” address beingcarried.• ar$pro.type – Protocol Type. This field is a 16 bit unsignedinteger.• ar$pro.snap - When ar$pro.type field equals to0x0080, a snap encoded extension, which is placedin the ar$pro.snap field. is used to encode the protocoltype. By default this field should be set to 0.• ar$hopcnt - Hop count- the maximum number ofNHSs that an MPOA packet is allowed to traverse.• ar$pktsz - The total length of the MPOA packet inoctets.• ar$chksum - The standard IP 16-bit checksum overthe entire MPOA packet.• ar$extoff - This field identifies the existence and locationof MPOA extensions.• ar$op.version - Version of generic address mappingand management protocol, set to X”01” NHRP• ar$op.type - The MPOA packet type. Some valuesfor packet types are:128130MPOA Cache ImpositionRequest.MPOA Egress CachePurge Request.129131MPOA Cache ImpositionReply.MPOA Egress CachePurge Reply.132 MPOA Keep-Alive. 133 MPOA Trigger.134 MPOA Resolution Request. 135 MPOA Resolution Reply.