MPOA integrates LANE and NHRP to preserve the benefits of LAN Emulation. MPOA allows the physical separation of network layer route calculation and forwarding. The mechanism is know as Virtual Routing. This separation provides a high performance and efficient inter-subnet communication over ATM Switched Virtual Channels (SVC) without requiring conventional routers in the data path. The separation increases manageability because it decreases the number of devices that must be configured to perform network layer route calculation. The separation reduces the number of devices participating in the network layer route calculation and that is why it increases scalability and reduces complexity.

Table of Contents

1 Introduction

2 Three Basic Elements

8. Conclusions

9 Glossary of Terms

10 References
 

1 Introduction

Nowadays internet and multimedia applications use great amounts of data and delay sensitive network traffic. People know that computer network speed is not at satisfactory level and more speed is needed all the time. Network managers install high-speed local area network (LAN) and Asynchronous Transfer Mode (ATM) switches to make their network more efficient. As the users send large amounts of data over subnet boundaries, the routers become as a bottlenecks for the network traffic.

Routers accept frames from one subnet and forward these frames to the hosts on other subnet. Routers has to do address resolution, route determination and packet filtering. All these tasks produce delay that makes speed lower. Data frames has probably to go through several routers and every router does same routing tasks and effect their own delay.

This slow routing would be solved if we could integrate routing with the ATM infrastructure. This solution should integrate the advantages of ATM with existing LAN technologies to preserve the investment in existing hardware and software.

The ATM Forum's LAN Emulation (LANE) emulates the services of Ethernet and Token Ring LANs across an ATM network. ATM network is divided into subnets known as Logical IP (Internet Protocol) subnets (LIS). LANE provides effective data transfer within a logical subnet but we need to forward data through routers to other logical subnets. [1]

The IETF's Next Hop Resolution Protocol (NHRP) makes NHRP queries between subnets to resolve ATM address of the destination (IETF or Internet Engineering Task Force). NHRP allows intermediate routers to be bypassed by establishing an ATM VCC (Virtual Channel Connection) across subnet boundaries. [1]

Even with LANE and NHRP there are LAN devices that are behind LANE edge devices. NHRP assumes that only routers and end-systems are attached to the LANE network. Edge devices, with virtual subnets, adds the complexity of needing to determine through which port of which edge device a particular end system may be reachable [5]. Multi-Protocol over ATM (MPOA) allows these edge devices to perform internetwork layer forwarding and establish direct communication without requiring that the LANE edge devices be full function routers.

The ATM Forum has developed in cooperation with the IETF a routing solution called MPOA. MPOA integrates existing protocols and introduce a concept know as a "virtual router". It provides routing functionality over switched ATM networks without requiring conventional routers in the data path. [2]

MPOA provides MPOA Clients (MPC) and MPOA Servers (MPS) and protocols to the communication between them [1].
 
 

2 Three Basic Elements

MPOA uses three basic elements to build a fundamental system. These elements or techniques to be exact are ATM Forum's LANE, the IETF's NHRP and the Virtual Router. [2]

2.1 LANE

LANE supports traditional internet protocols over an ATM subnet in a transparent manner. It emulates the services of Ethernet and Token Ring LANs and enables all existing LAN applications to run over ATM. LANE is used for intra-subnet communications, while the MPOA Virtual Router provides communication between subnets. LANE Version 2 is an integral component of MPOA.

2.2 NHRP

Normal routing protocols summarize information so that details are hidden. Most routing protocols do not include layer 2 information that is needed to establish a VCC connection. NHRP allows the intermediate routers on the data path to be bypassed. It provides an extended address resolution protocol that permits Next Hop Clients (NHC) to send queries between different logical subnets.

2.3 Virtual Router

A Virtual Router is a set of devices that provide routing functions. These devices are the MPOA Server and the edge devices. The MPOA defines also protocols between them.
 
 

3 Components

MPOA defines components like Edge Device and MPOA router. Vendors can implements these components in various ways that meet the needs of their customers.
 

3.1 Edge Device

An edge device is a physical device that forwards data between traditional LAN and ATM based on the destination network address and medium access control (MAC) layer address. It can contain one or more ATM interfaces and legacy ports. [4] The edge device contains an MPC and a LAN emulation Client (LEC). The MPC sets up a virtual connection and forwards data to the destination across the path.

3.1.1 MPC

An MPC acts as a point of entry and exit for internetwork shortcuts. It monitors the layer-3 traffic and requests and initiates shortcuts for flows. An MPS provides needed information on the destination and if a shortcut is acceptable, the MPC sets up a VCC and forwards data to the destination. The MPC caches the shortcut information that it gets from the MPS. The VCC will expire if there is no activity within a fixed time. [2]

3.1.2 LEC

The LEC is specified to be a client on a particular emulated LAN (ELAN). The LEC is responsible for data forwarding, address resolution, and other control functions for a single end-system within a single ELAN. A standard LAN service interface is provided to any higher layer entity which interfaces the LEC. [3] Each LEC is identified by a unique ATM address, and is associated with one or more MAC addresses reachable through that ATM address.

3.2 MPOA Router

The MPOA Router allows mapping of network layer subnets to ATM. It contains an MPS, Next Hop Server (NHS) and LEC. The MPOA Router maintains MAC-layer and ATM address information in addition to routing tables. It interoperates with existing routed LAN and Wide Area Network (WAN) internetworks.

3.2.1 MPS

The MPS has interfaces with the router for layer 3 information and interfaces with LANE for layer 2 information. It communicates with other MPOA devices to resolve layer 3 addresses to ATM addresses. [4]

3.2.2 NHS

Each NHS maintains "next-hop resolution" cache tables with IP to ATM address mappings of all those nodes associated with that particular NHS or for IP address prefixes reachable through nodes (routers) served by the NHS. Nodes are configured with the ATM address of their NHS and then register their own ATM and IP addresses with the NHS so that the NHS can build its cache tables. [5]
 
 

4 The flows of MPOA

There are two types of information flows in MPOA. They are MPOA control flows and MPOA data flows.

All control and data flows are carried over ATM VCCs using LLC/SNAP [15] encapsulation. Configuration flows use the formats described in [4].

4.1. The control flows of MPOA

There are four kinds of control flow type in the MPOA:

- configuration flows

- control flows between MPC and MPS

- control flows between MPS and MPS

- control flows between MPC and MPC

Configuration flows are used between MPSs and MPCs. They communicate with the LAN Emulation Configuration Server (LECS) to fetch configuration information.

Control flows between MPC and MPS are used for MPC cache management. The MPOA Resolution Request and Reply mechanism allows the ingress MPC to obtain shortcut information [1].

4.2. The data flows of MPOA

There are two kinds of data flow type in the MPOA:

- the data flow between MPC and MPC

- the data flow between MPC and NHS

The data flow between MPC and MPC are used principally for the transfer of data between MPCs over MPOA shortcut VCCs.

An MPC can send data to an NHC and vice versa.
 
 

5 MPOA Operations

MPOA performs the following operations: Configuration, Discovery, Target Resolution, Connection Management and Data Transfer.

5.1 Configuration

MPCs and MPSs needs configuration. MPOA components fetch their configuration parameters from the LECS. MPOA components may be administered to get their configuration by some other means.

5.2 Discovery

Then MPOA components are operating, they tries to find out each other. MPOA components sends the LANE LE_ARP_REQUEST-messages that carry the MPOA device type (MPC or MPS) and ATM address to each other.

5.3 Target Resolution (creating the shortcut)

MPOA builds shortcut connections over ATM from any MPOA capable host or edge device to any other regardless if their subnet memberships. Edge device is the optimal exit from the ATM cloud. This edge device forwards data further to the destination host or conventional router if necessary. MPOA ensures compatibility with the existing infrastructure of routers.

The shortcut connection provides a remarkable improvement in performance and end to end delay compared to pure hop by hop router based solution.

An MPC searches possible shortcuts and performs internetwork layer forwarding. When it recognizes a flow that could benefit from a shortcut, it uses an NHRP protocol to request the information from the MPS required to establish a shortcut to the destination. The name of the request is an MPOA Resolution Request and answer to this is an MPOA Resolution Reply. The MPOA Resolution Reply contains the ATM address for the destination of a shortcut. If a shortcut is possible to do, the MPC sets up a shortcut VCC and forwards data over the shortcut to the other MPC. The MPC that forwards data is called the ingress MPC and destination MPC is called the egress MPC [1].

A packet enters the MPOA System at the ingress MPC (e.g. MPC 1). The decision process that takes place at an MPC is outlined in Figure 2. The packet is bridged via LANE to a router. If the packet follows the default path, it leaves the MPOA System via the ingress MPC's internal LEC Service Interface.
 
 

If no flow has been detected previously, each packet being sent to an MPS is tallied by internetwork layer destination address as it is being sent via LANE. When a threshold (given as a number of packets for a single internetwork layer address in a fixed period of time) is exceeded, the MPC is required to send an MPOA resolution request to obtain the ATM address to be used for establishing a shortcut to a specific downstream element - most likely an egress MPC (e.g. MPC 2).olution Reply to the ingress MPC.

On arriving via shortcut at the egress MPC, a packet is examined and either a matching egress cache entry is found or packet is dropped. If a match is found, the packet is encapsulated using the information in the egress cache, and it is forwarded to the higher layer.
 

There are also two types of MPSs. The ingress MPS is in the ingress MPC side and the egress MPS is in the egress MPC side. An ingress MPS provides internetwork layer forwarding information to MPCs as it is depicted in the figure 3. The ingress MPS interacts with its local NHS and routing functions to give a reply to MPOA queries. If the destination MPC is not local, the MPS re-originates the request along the routed path through the destination MPC's local NHS. On receiving a reply to this re-originated request the ingress MPS returns an MPOA Resolution Reply to the ingress MPC.

The egress MPS receives queries from the ingress MPS. These queries are called NHRP Resolution Requests and the egress MPS sends a reply called NHRP Resolution Reply. The egress MPS needs more information before it can formulate an NHRP Resolution Reply. To obtain this information it sends an MPOA Cache Imposition Request to the egress MPC and gets back an MPOA Cache Imposition Reply. The MPOA Cache Imposition Request provides also encapsulation and state maintenance information needed by the egress MPC and the MPOA Cache Imposition Reply provides status, address and ingress tagging information.

The egress MPC must determine if it has enough resources to maintain the cache entry and receive a new VCC. If it has the necessary resources, the MPC inserts an ATM address to the cache and sends the MPOA Cache Imposition Reply to the egress MPS.

MPCs uses an NHRP Resolution Request protocol to find out the ATM address for the end points of a shortcut.

5.4 Connection Management

MPOA components establish VCCs between each other. VCCs are established when it is necessary to transfer data and control messages over an ATM network. MPOA components maintains and terminates VCCs also.

5.5 Data Transfer

The main task of MPOA is the efficient data transfer. There are two operation modes in the MPOA System: the default flow and the shortcut flow. The default flow follows the routed path over the ATM network. In the default case, the MPOA edge device acts as a layer 2 bridge. There are needed the MPOA target resolution and cache management mechanisms to establish Shortcuts.

If an MPC has an Internetwork protocol packet to send for which it has a shortcut, when the MPOA edge device acts as an internetwork level forwarder and sends the packet over the shortcut.
 
 

6. MPOA and other routing protocol interaction

There are two side in the system based on MPOA. These sides are Host and Network side. In the Host side it is compared standard TCP/IP (Transport Control Protocol) connection with TCP/IP connection over MPOA. In the Network side it is compared standard IP router with MPOA. It is described in the following two figures how MPOA is situated in the protocol stack. It is shown in the figure 4 that in the Host side MPOA needs LANE, Q.2931 and SSCOP protocol stacks to communicate with other side. In the figure 5 it is described network side. In the Network side MPOA needs PNNI, Q.2931 and SSCOP protocol stacks to communicate with other side. In the figure 4 there is TCP/IP and TCP/IP over MPOA comparison in the Host side. In the figure 5 there is IP Router and MPOA comparison in the Network side.

Host

Figure 4. TCP/IP and TCP/IP over MPOA comparison in the host side.

Network
 
 

Figure 5. IP Router and MPOA comparison in the network side.

PNNI (Private Network-to-network Interface or Private Network Node Interface), Q.2931 and SSCOP (Service Specific Connection Oriented Protocol) protocols are described in the following chapters.

6.1 SSCOP

SSCOP - Defined in [6], the Service Specific Connection Oriented Protocol is responsible for providing mechanisms for the establishment, release and monitoring of signalling information exchanged between peer signalling entities.

SSCOP provides many of the same services for Q.2931 signalling that TCP provides for IP. Most important of these services are:

• Flow control. A switch or end system can control the rate at which it receives signalling messages.
• Error handling and sequencing. Since each SSCOP PDU (Protocol Data Unit) contains a sequence number, SSCOP can determine if one has been lost and request retransmission. SSCOP can also ensure sequential integrity by using these sequence numbers.
• Connection establishment and resynchronization. Involves negotiation of buffer sizes and other transfer characteristics and renegotiation of these parameters should a connection fail.
• Polling and status exchange. The health of a connection is monitored via exchange of status messages and connections are maintained through use of keep alive messages.

Q.2931 - Defined in [7], is responsible for providing mechanisms for the establishment, maintaining, and clearing of network connections at the Broadband Integrated Services Digital Network (B-ISDN) user-network interface.

Q.2931 is intended to specify the essential features, procedures, and messages required for call/connection control.

Q.2931 specifies the layer 3 call/connection states, messages, information elements, timers, and procedures used for the control of B-ISDN point-to-point on demand calls on virtual channels.

6.3 PNNI

PNNI is defined in [8] and it is called both Private Network-to-network Interface and Private Network Node Interface. The PNNI protocol is a source routing protocol. Source routing has the entire path specified by the source node. The path information is included in the call setup message, and signalling follows the path accordingly.

Features of PNNI

• Suitable for point-to-point and point-to-multipoint connections
• Because it has multiple levels of hierarchy, it is scaleable for global networking.

7. Commercial examples of MPOA devices

Newbridge networks has a MPOA-based VIVID product. Based on the ATM Forum MPOA specification, the VIVID Route Server offers optimized route calculation, bridged and routed Virtual LAN (VLAN) configuration, and secure network-wide host admission control. Robust mechanisms are used to distribute policy information to all appropriate network devices, and transfer control to secondary systems in the event of a failure [13].

FORE Systems Inc. launched the PowerHub 8000 Multilayer Switch that now supports MPOA. Its backplane scales to 3.2Gbps and supports throughput of more than 1 million packets per second. With MPOA the unit can separate the data-forwarding function from the control path. It supports as many as 128 Ethernet ports, 84 fast-Ethernet ports, 16 FDDI rings, and four ATM ports. Up to 226 workstations can be connected to the $15,950 unit [11].

Newbridge Networks and Interphase [12] introduce MPOA-based PCI NIC for VIVID switched routing solution. The Interphase PCI NIC, which includes drivers for Windows NT and Novell NetWare, provides both Layer-2 (MAC-layer) bridging and Layer-3 (network-layer) routing in a VIVID ATM networking environment. The drivers also support Newbridge's NATM commands and the VIVID System Manager, allowing seamless monitoring and control of VIVID networks. In addition, the Interphase PCI NIC has been extensively tested for interoperability with the VIVID NICs, Ridges and Route Servers.

Alcatel Data Networks announces that the 1100 LSS(R), a LAN and campus backbone switch, will support MPOA, the network layer switching standard recently ratified by the ATM Forum. MPOA defines a high-performance, low-latency model for supporting Internet Protocol (IP) traffic over ATM in campus networks. The Alcatel 1100 LSS will be MPOA-enabled in the fourth quarter of this year [9].

The PLOA (Protocol Layers Over ATM) software product provides a modular implementation of multiple protocol layers over ATM. PLOA is used as a vehicle to support IETF's Classical IP over ATM (CIP), ATM Forum's MPOA, and ATM Forum's LANE 2.0 variant. Supports full implementation of IETF's Classical IP over ATM (RFC 1577) - Server and Client. MPC, MPS, LAN Emulation Client 2.0 and LAN Emulation Services 2.0 are currently under development [14].

Bay Networks introduce high-speed riser links extend from the network center to high-speed Centillion 1400 or 1200 switches, providing switch aggregation in large campus environments. Risers also provide connectivity to Centillion 1200 switches supporting power workgroups, as well as to high-density System 5000BH, Centillion 100, or Centillion 50 LAN-ATM edge switches deployed in wiring closets across the network. These edge switches provide full integration between the ATM backbone and existing Ethernet and Token Ring LANs via LANE. By adhering to the ATM Forum's MPOA standard, the Centillion architecture delivers direct Layer 3 interconnection between switches, eliminating traffic latencies at the router. The fully-meshed topology provides an extremely fault-tolerant, resilient architecture capable of fast failover in the event of a component failure, preserving network viability even under adverse operating conditions [10].

8. Conclusions

It is clear that a specific model for integrating ATM into today's multiprotocol is needed in a way that permits organizations to build scalable and manageable multi-media internetworks, retaining the important functionality of routers while allowing the continued use of existing Ethernet, Token Ring, and TCP/IP and SPX/IPX (Sequenced Packet Exchange Protocol Stack / Internetwork Packet Exchange) infrastructures. MPOA integrates LANE and NHRP to preserve the benefits of LAN Emulation, while allowing inter-subnet network layer protocol communication over ATM Switched Virtual Channels (SVC) without requiring conventional routers in the data path. MPOA allows the physical separation of network layer route calculation and forwarding, a technique known as Virtual Routing. This separation provides a number of key benefits:

• High performance and efficient inter-subnet communication.
• Increased manageability by decreasing the number of devices that must be configured to perform network layer route calculation.
• Increased scalability by reducing the number of devices participating in the network layer route calculation.
• Reduced complexity of the Edge Devices by eliminating the need to perform network layer route calculation.

9 Glossary of Terms

ATM Asynchronous Transfer Mode

B-ISDN Broadband Integrated Services Digital Network

CIP Classical IP over ATM

ELAN Emulated LAN

FDDI Fiber Distributed Data Interface

IETF Internet Engineering Task Force

IP Internet Protocol

IPX Internetwork Packet Exchange

LAN Local Area Network

LANE LAN Emulation

LEC LAN emulation Client

LECS LAN Emulation Configuration Server

LIS Logical IP Subnets

LLC Logical Link Control

MAC Medium Access Control

MARS Multicast-Address Resolution Server

MPC MPOA Clients

MPOA Multi-Protocol over ATM

MPS MPOA Server

NHC Next Hop Client

NHRP Next Hop Resolution Protocol

NHS Next Hop Server

PDU Protocol Data Unit

PLOA Protocol Layers Over ATM

PNNI Private Network-to-network Interface or Private Network Node

Interface

SNAP SubNetwork Attachment Point

SPX Sequenced Packet Exchange

SSCOP Service Specific Connection Oriented Protocol

SVC Switched Virtual Channel

TCP Transport Control Protocol

UDP User Datagram Protocol

VLAN Virtual LAN

VCC Virtual Channel Connection

WAN Wide Area Network

10 References

[1] The ATM Forum Technical Committee. Multi-Protocol Over ATM Specification Version 1.0, July 1997.

[2] Gerald P. Ryan. MPOA Multiprotocol over ATM. <http://www.siemens.se/siemensab/kt/itnet/papers/mpoa/mpoa.html> The Applied Technologies Group, 1997.

[3] Andrew Cobden. LAN Emulation. <http://bugs.wpi.edu:8080/EE535/hwk96/hwk5cd96/cobden/cobden.html> Proceedings of Telecommunications Transmission Technologies Course Worcester Polytechnic Institute, 1996.

[4] The ATM Forum. LAN Emulation and MPOA, presentation sheets, 1996.

[5] Anthony Alles. ATM Internetworking. <http://www-tkn.ee.tu-berlin.de/~festag/atm_tutorial/12.html> Cisco Sytems inc., June 1995

[6] ITU Q.2110, B-ISDN SAAL Service-Specific Connection-Oriented Protocol SSCOP.

[7] ITU Q.2931, the Broadband Integrated Services Digital Network (B-ISDN) Digital Subscriber Signalling System No. 2 (DSS 2) User Network Interface Layer 3 Specification for Basic Call/Connection Control.

[8] ATM Forum 94-0471R9, "PNNI Draft Specification.

[9] Alcatel Data Networks. <http://www.adn.alcatel.com>

[10] Bay Networks. <http://www.baynetworks.com>

[11] FORE Systems Inc. <http://www.fore.com>

[12] Interphase. <http://www.iphase.com>

[13] Newbridge Networks. <http://www.vivid.newbridge.com>

[14] Trillium Digital Systems. ;<http://www.trillium.com>

[15] Multiprotocol Encapsulation over ATM Adaptation Layer 5. RFC 1483.