metro future

MPLS, RPR AND ASON IN THE METRO

- A UNIFIED FUTURE

Abstract Metropolitan Area Networks (MANs) are defined as networks spanning distances up to several hundred kilometers, typically serving large, concentrated metropolitan areas. Current metro-area network topologies are largely ring-based. SONET/SDH is the technology used in the metro area, using point-to-point or add-drop multiplexer (ADM) ring topologies. Connections are either permanent or semi-permanent with access rates ranging from OC-3 to OC-48. Metro networks present many engineering challenges, especially as there is a large base of legacy SONET/SDH (Synchronous Optical Network/Synchronous Digital Hierarchy) infrastructure prevalent in current metro-area networks. These traditional TDM (time-division multiplexing) networks were originally designed to transport a limited set of traffic types, mainly multiplexed voice and private line services (such as DS-1 and DS-3). Today's metro market is under pressure to handle the rapidly growing capacity demands and increasingly varying traffic patterns. The increase in long-haul DWDM capacity coupled with the rise of (access) IP bandwidth demand has placed a focus on the metro network to provide additional capacity. This paper tries to envision a metro network where technologies as Multi Protocol Label Switching (MPLS), Resilient Packet Ring, (RPR) and Automatically Switched Optical Network (ASON) would work together and remove bottlenecks and streamline network efficiency. It begins with a brief description of a typical current metro network, the technologies used and typical metro provider requirements. A brief description of the newer technologies that are being considered follows along with their individual advantages, a unified network where all these technologies co-exist and work in unison and the inherent advantages and disadvantages of such a network. Introduction Metropolitan Area Networks bridge the space between long haul and access networks, interconnecting a full-range of client protocols from enterprise/private customers in access networks to backbone service provider networks. A typical metro network can be categorized as follows. • A regional/metro core transport ring that is typically SONET/SDH based at bandwidth rates

from OC-12 to OC-48 with DWDM being starting to make an appearance. This ring is used connect the cities or large urban concentrations in a larger metropolitan area. This is also called the metro backbone. The backbone typically connects to a long haul network for interconnection between different MANs.

• A citywide aggregation network that feeds the metro backbone. This is typically a TDM or a

leased line network that runs at rates from T1 to DS-3. This is also called the metro edge that indicates the interface between the metro and the access network. The access network rates span a broad spectrum like T1, DS-3, 10 Mb/s Ethernet, OC-3 and OC-12 that the metro network grooms and uses in intra-metro as well as inter-metro connectivity.

The challenges that a metro network provides for service providers are

Service Breadth where the providers must be able to offer a variety of offerings like IP, Frame Relay, Ethernet, ATM etc with flexibility for new services without heavy additional cost. Service Delivery where the network is optimized for changing access requirements with easy and quick provisioning.

MPLS, RPR and ASON in the Metro – A Unified Future 2

Service Awareness where requirements as Quality of Service, Class of Service, Protection levels take higher precedence. Scalability where the network capacity should be able to scale much higher than the capital costs for upgrades.. Network Reliability where the capability of a SONET network must be maintained with regards to protection and restoration. Co-existence with existing infrastructure to carry voice as well as data traffic as well as interfacing with existing infrastructure Topological flexibility where the equipment must be able to support different physical topologies thereby supporting flexible application traffic flows. Reduction in operational and network costs since the metro network is driven by central office access and transmission equipment costs. Newer equipment must offer increased functionality and performance without proportional cost increase.

The New Technologies Multi-Protocol Label Switching Multi-Protocol Label Switching (MPLS) was developed as a packet-based technology and is rapidly becoming key for use in core networks, including converged data and voice networks. MPLS does not replace IP routing, but works alongside existing and future routing technologies to provide very high-speed data forwarding between Label-Switched Routers (LSRs) together with reservation of bandwidth for traffic flows with differing Quality of Service (QoS) requirements. MPLS uses a technique known as label switching to forward data through the network. A small, fixed-format label is inserted in front of each data packet on entry into the MPLS network. At each hop across the network, the packet is routed based on the value of the incoming interface and label, and dispatched to an outgoing interface with a new label value. The path that data follows through a network are defined by the transition in label values as the label is swapped at each LSR. Since the mapping between labels is constant at each LSR, the complete path is determined by the initial label value. Such a path is called a Label Switched Path (LSP). A set of packets that should be labeled with the same label value on entry to the MPLS network, and that will therefore follow the same LSP, is known as a Forwarding Equivalence Class (FEC).

Figure 1: A MPLS Network in action

IP 21

IP 17

IP IP 47

IP 11 IP

IP

Host Y LSR C

LSR B

LSR A

Host Z

LSR D

Host X

IP


The figure shows two data flows from host X: one to Y, and one to Z. Two LSPs are shown. LSR A is the ingress point into the MPLS network for data from host X. When it receives packets from X, LSR A determines the FEC for each packet, deduces the LSP to use and adds a label to the packet. LSR A then forwards the packet on the appropriate interface for the LSP. LSR B is an intermediate LSR in the MPLS network. It simply takes each labeled packet and uses the pairing {incoming interface, label value} to decide the pairing {outgoing interface, label value} with which to forward the packet. This procedure can use a simple lookup table and can be performed in hardware, along with the swapping of label value and forwarding of the packet. In the example, each packet with label value 21 will be dispatched out of the interface towards LSR D, bearing label value 47. Packets with label value 17 will be re-labeled with value 11 and sent towards LSR C. LSR C and LSR D act as egress LSRs from the MPLS network. These LSRs perform the same lookup as the intermediate LSRs, but the {outgoing interface, label value} pair marks the packet as exiting the LSP. The egress LSRs strip the labels from the packets and forward them using layer 3 routing. An LSP can be established that crosses multiple Layer 2 transports such as ATM, Frame Relay or Ethernet. Thus, one of the true promises of MPLS is the ability to create end-to-end circuits, with specific performance characteristics, across any type of transport medium, eliminating the need for overlay networks or Layer 2 only control mechanisms. The exact format of a label and how it is added to the packet depends on the layer-2 link technology used in the MPLS network. For example, a label could correspond to an ATM VPI/VCI, a Frame Relay DLCI. For other layer 2 types (such as Ethernet and PPP) the label is added to the data packet as a MPLS shim header, which is placed between the layer 2 and layer 3 headers. In a similar way, a label could correspond to a fiber, a DWDM wavelength, or a TDM timeslot. A generalized label has been proposed for extending this concept into optical networks to encompass TDM, wavelength switching (lambdas) and spatial switching (at a fiber level) and is called Generalized Multi-Protocol Label Switching (GMPLS) The benefits offered by a MPLS based network are as follows.

• Simplified and fast forwarding • Separation of routing and forwarding in IP networks

- facilitates evolution of routing techniques by fixing the forwarding method - new routing functionality can be deployed without changing the forwarding

techniques of every router in the Internet • Traffic Engineering

- Constraint-based Routing • Virtual Private Networks

- Controllable tunneling mechanism Resilient Packet Rings (RPR) Metropolitan and WAN networks are widely deployed on an optical ring network. These rings use protocols that are neither optimized nor scalable to the demands of packet networks that include speed of deployment, bandwidth allocation and throughput, resiliency to faults and reduced equipment and operational costs. RPR provides advantage over those protocols by removing the shortcomings of those protocols. RPR is a Media Access Control protocol providing a scalable LAN/MAN/WAN architecture with a shared access method, spatial re-use, resiliency through fault protection, scalability across a large number of stations and dynamic topology learning. It also provides media independent service interface from MAC to PHY layer. RPR uses a bi-directional ring. This can be seen as two symmetric counter-rotating rings.


A node on a ring needs to do three packet handling operations, viz. adding a packet into the ring, forwarding a packet and taking a packet off the ring. This reduces the amount of work that individual nodes have to do compared to a mesh network where each node has a lot of decisions to make before forwarding. In addition, the assumption of a ring topology allows better resiliency, multicasting, and bandwidth sharing as compared with a broadcast media as Ethernet. RPR technology is key for integration of legacy and Ethernet technologies as they interact with existing SONET or DWDM transport deployments. A brief description of the parameters associated with a RPR follows. Fairness

RPR uses a fairness algorithm to regulate the bandwidth usage by each node by ensuring fair usage of the ring bandwidth thereby maximizing ring utilization. If congestion is experienced in a node, congestion intimation will be sent to the upstream node so that node may throttle its rate of packet transmission. This is an easy method of preventing neighboring nodes from acting as ‘bandwidth hogs’.

Resiliency

RPR, through Intelligent Protection Switching recovers fully from any disturbance like fiber cut within 50ms of its occurrence by wrapping data packets away from the failed span. The wrapped data can reach the destination by going around the ring in opposite direction. The following figure shows a typical ring protection in action. Topology discovery will discover the new topology as soon as a wrap happens so that new optimal path for all nodes in the ring will be learnt.

Figure 2: RPR protection illustrated

RPR will

• Support a minimum data rate of 155Mb/s, scalable to higher speeds. • Support for dual counter rotating ring over fiber optic. • Efficient use of bandwidth by the use of spatial reuse and minimal protocol

overhead. • Support for three traffic classes. • Scalability across a large number of stations attached to a ring • "Plug and play" design without a software based station management transfer

(SMT) protocol or ring master negotiation as seen in other ring based MAC protocols.

• Fairness among nodes using the ring (Each station can be assigned a proportion of the ring bandwidth).

• Support for ring based redundancy (error detection, ring wrap, etc.) • Provide media independent service interface from MAC to PHY layer.


Automatically Switched Optical Networks (ASON) The existing transport networks that provide SONET/SDH and WDM services have connections that are provisioned via network management protocols. This process is both slow (weeks to months) and proves costly to the network providers. The growing trend of data traffic is also posing challenges not only in terms of volumes but also related to the burst and asymmetrical nature of such traffic. The emergence of enterprise networking and end-user applications causes abrupt fall and rise in bandwidth demand. The transport networks should fulfil new emerging requirements such as fast and automatic end-to-end provisioning, optical re-routing and restoration, support of multiple clients, deployment of Optical Virtual Private Networks (OVPN) and interworking of IP-based and Optical Transport Networks. An Automatically Switched Optical Network meets these given requirements. The major features associated with ASON can be listed as follows.

• Quick and Dynamic end-to-end connection management to create end-to-end connections of different connection types and granularity, Delete connections, Modify Connections and Status enquiry of connections. Neighbor discovery to provide automatic discovery of the physical interfaces and properties of directly connected routers or cross-connects and Service discovery to provide automatic discovery of the services available over a UNI.

• Different path computation mechanisms based on service classes with the information from neighbor discovery and service discovery.

• Path restoration mechanisms in case of link failures with localization of faults and application of different restoration mechanisms.

• Client-driven provisioning functionality for addition/deletion of bandwidth. Upon request, automatic provisioning is done without manual intervention.

The optical transport network (OTN) needs to provide a User-To-Network Interface (UNI) that allows client network devices to request connections across it dynamically. An ASON may be typically realized using a GMPLS based control plane for signaling. The ASON Control Plane defines a set of interfaces:

- User-Network Interface (UNI): UNI runs between the optical client and the network. - Internal Node-to-Node Interface (I-NNI): I-NNI defines the interface between the signaling

network elements. - External Node-to-Node Interface (E-NNI): E-NNI defines the interface between ASON control

planes in different administrative domains.

Figure 3: An architecture diagram of an ASON


Coming together, the Unified Network Metro networks are currently predominantly SONET based rings with mesh networks also making an appearance because of their advantages. For quite some time these two different network architectures will co-exist as they both have advantages and disadvantages. The figure below shows how various technologies that are being discussed will work together to achieve a unified solution for the Metro Networks. This network uses existing optical infrastructure that was primarily designed for voice traffic (SONET) and enhances the same for carrying data. It also assumes a mesh-based architecture for the core, which probably would be the architecture of the future considering a proliferation of DWDM into the metro area.

The optical transport network is ASON enabled using GMPLS. This optical network will provide dynamic and fast provisioning of connections as and when requested. A RPR can be configured over this transport network as shown in the figure. A virtual ring is formed from a setup of LSPs through the network connecting the RPR nodes. The RPR nodes get input from the MPLS enabled routers connected to the enterprise network. RPR will enhance the bandwidth utilization and provides effective protection to deal with any failure or faults occurring in the RPR ring. The route through which the RPR traffic is transported over the transport network will be transparent to the RPR nodes. An edge node of a TDM network element connected to the transport network as in the figure can request for bandwidth from the core optical network dynamically when configured as an UNI client. The GMPLS enabled optical network creates an LSP from the source node to the destination for the requested bandwidth and appropriate QoS. A typical example of the use of an UNI is for the connecting a network data center and an enterprise. The enterprise may configure to backup the data at pre-determined intervals and the enterprise router which is UNI enabled can request the transport network for additional bandwidth to the associated router in the data center for transfer of information. When the transfer is complete, the enterprise router may request a deletion of the connection thereby releasing network bandwidth that may otherwise not be used until the next transfer. This is an example of a typical bandwidth-on-demand service that can be realized by using this network.

Client device - Router

Enterprise Network

RPR enabled router GMPLS enabled OXC / SONET node

RPR based ring through the network

UNI

UNI

MPLS

Inter metro Connectivity

MPLS Core router

DWDM

UNI

GMPLS

GMPLS GMPLS

GMPLS

GMPLS MPLS

LONG HAUL NETWORK

LONG HAUL

UNI

RPR

RPR

RPR

RPR

SONET NETWORK

GMPLS

ADJACENT METROPOLITAN

AREA

Figure 4: The unified future network


Benefits of the Unified solution • Better utilization of the bandwidth in the network. • Faster and dynamic connection provisioning because of less manual intervention. • Rapid fault restoration based on different classes of service and priorities. • Scalability. The network can be re-organized based on client needs. • Coexistence with already existing legacy infrastructure. • Reduction in the operational cost. Complexities involved • Network management will be complex due to the different technologies and their associated

architectures and existence of the multiple level control plane. • RPR doesn’t lend itself to traffic engineering well as it always takes the shortest path available

from source to destination irrespective of the traffic load. Summary The paper tries to project a typical metro network into the future when all the technologies come into play. MPLS is already established in the network and RPR and ASON are expected to be in the network in a short span of time. Integration of DWDM in the metro will provide more value to the provider in terms of increase in network utilization for a relatively lower establishment cost. Glossary

ADM Add Drop Multiplexer, a device that adds and drops digital/optical signals in a typical ring based network.

ASON Automatically Switched Optical Network DS-3 Digital Signal Level 3 (44.7 Mbps, data rate, 672 voice channels) DWDM Dense WDM, technology that puts data from different sources together

on an optical fiber, with each signal carried at the same time on its own separate light wavelength

E-NNI External NNI, interface between two networking subnetwork domains GMPLS Generic Multi-Protocol Label Switching I-NNI Internal NNI interfaces between networking elements in the same

subnetwork domain. LDP Label Distribution Protocol, a protocol to distribute label information

between MPLS peer routers MAC Media Access Control is a data link sub-layer which is different for each

physical device and controls access to the device. MAN Metropolitan Area Networks MON Metropolitan Optical Network MPLS Multi-Protocol Label Switching is technology for speeding up network

traffic flow by switching at Layer 2 rather than looking up and routing at Layer 3 of the OSI stack.

NNI Network-Network Interface OC-3/OC-48 Optical Carrier 3/48 ( 155 Mbps/2.48 Gbps) An optical signal rate

typically used in metros. OTN Optical Transport Network RPR Resilient Packet Ring SDH Synchronous Digital Hierarchy, International standard technology for

synchronous data transmission on optical media SMT Station Management Transfer SONET Synchronous Optical NETwork, a North American based technology for

synchronous data transmission on optical media TDM Time Division Multiplexing, a scheme in which numerous signals are

combined for transmission on a single communications line or


UNI User Network Interface, the interface between a client device and the network device.

References/Acknowledgements 1) How Ethernet, RPR, and MPLS work together: The Unified Future of Metro Networking

By Tim Wu, Riverstone Networks. 2) RFC 2892 – Resilient Packet Ring 3) MPLS in Optical Networks : An analysis of the features of MPLS and GMPLS and their application

to Optical Networks ,with Reference to the Link Management Protocol and Optical UNI by Neil Jerram and Adrian Farrel

4) Metropolitan Optical Networks: Overview and Requirements by Sorrento Networks. About the authors Gayathri Manoj, Kandasamy Varadharaj, Shri Krishan, RamNarayan S are design engineers at Wipro Technologies and work with various technologies across different domains ranging from voice switching, routers, network management and optical ADMs. Wipro provides Research and Development services to Telecom and Electronic product companies and software solutions to global corporate enterprises. In the Indian market, Wipro is a leader in providing IT solutions and services for the corporate segment offering system integration, network integration and IT services. Wipro Limited is the first SEI CMM Level 5 certified IT Services company operating in the global market. Wipro Technologies can be reached on the Web via the URL http://www.wipro.com

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