Multi-protocol Label Switching (MPLS)
Enterprises are choosing Ethernet as their wide area service due to its affordability, scalability, and flexibility to meet a wide range of networking requirements. Ethernet is a great fit for Cloud Computing. Operationally it’s a simple value proposition: plug-in into the cloud, select the data rate to meet your needs, and go! If you need more bandwidth just dial it up. No more long waits for truck rolls, equipment upgrades, or complex provisioning processes.
In many Cloud deployments, carriers use Ethernet as the high bandwidth access to an MPLS network in the carrier backbone, and MPLS transparently switches the Ethernet traffic to its destination. MPLS enables highly predictable performance, hard QOS, and the flexibility to carry multi-protocol traffic. Distance can be metro, regional, national, and global. For the major carriers, MPLS delivers the any-to-any switching with Ethernet as the affordable high speed “on ramp” to their MPLS network.
The key thing to remember about MPLS is that it’s a technique, not a service — so it can be used to deliver anything from IP VPNs to Carrier Ethernet services, or even to provision optical services. So although carriers build MPLS backbones, the services that users buy may not be called MPLS. They could be called anything from IP VPN to Metro Ethernet — or whatever the carriers’ marketing departments create next.
The fundamental concept behind MPLS is that of labeling packets. In a traditional routed IP network, each router makes an independent forwarding decision for each packet based solely on the packet’s network-layer header. Consequently, every time a packet arrives at a router, the router has to buffer and “think through” where to send the packet next.
The idea is to steer IP traffic onto a variety of routes instead of the single one discovered by an interior gateway protocol such as Border Gateway Protocol (BGP), to avoid congestion or failures, or to enable a particular class of service or guaranteed service level. This crucial for applications such IP video which demand low latency and predictable performance.
MPLS switches and routers affix labels to packets based on their destination, type-of-service parameters, Virtual Private Network membership or other criteria. As a packet traverses a network, other switches and routers build tables associating packets and routes with labels. The MPLS switches and routers - dubbed label switch routers - assign each packet a label that corresponds to a particular path through the network.
All packets with the same label use the same path - a label switched path (LSP). Because labels refer to paths and not endpoints, packets destined for the same endpoint can use a variety of LSPs to get there.
With MPLS, the first time the packet enters a network, it’s assigned to a specific forwarding equivalence class (FEC), indicated by appending a short bit sequence (the label) to the packet. Each router in the network has a table indicating how to handle packets of a specific FEC type, so once the packet has entered the network, routers don’t need to perform header analysis. Instead, subsequent routers use the label as an index into a table that provides them with a new FEC for that packet.
This gives the MPLS network the ability to handle packets with particular characteristics (such as coming from particular ports or carrying traffic of particular application types) in a consistent fashion. Packets carrying real-time traffic, such as voice or video, can easily be mapped to low-latency routes across the network — something that’s challenging with conventional routing. The key architectural point with all this is that the labels provide a way to “attach” additional information to each packet — information above and beyond what the routers previously had.
Comparison to OSI Model
There’s been a lot of confusion over the years about whether MPLS is a Layer 2 or Layer 3 service. MPLS doesn’t fit neatly into the OSI seven-layer hierarchy. In fact, one of the key benefits of MPLS is that it separates forwarding mechanisms from the underlying data-link service. MPLS can be used to create forwarding tables for ATM or frame relay switches (using the existing ATM or DLCI header) or for plain old IP routers by appending MPLS tags to IP packets.
The business value is that network operators can use MPLS to deliver a wide range of lucrative services by seamlessly interworking the customer’s existing WAN network. The two most popular implementations of MPLS are Layer 3 BGP/MPLS-VPNs (based on RFC 2547) and Layer 2 (or pseudowire) VPNs.
RFC 2547 VPNs have been implemented by most of the major service providers, including AT&T, Verizon, BT and many others. The fundamental characteristics of a 2547 is that traffic is isolated into MPLS-VPNs as it enters the network.
Interior routers have no knowledge of IP information beyond the label-only base forwarding decisions on the MPLS label. BGP is used by edge routers to exchange knowledge of VPNs, thus enabling service providers to isolate traffic from multiple customers or even the Internet over a shared backbone.
There are several flavors of layer 2 MPLS services, but what they have in common is that a Layer 2 packet (example: Ethernet) is encased in an MPLS header and forwarded through the MPLS core. When it reaches the other side, the packet’s labels are removed, and the packet that arrives at the ultimate destination exactly as it entered the MPLS network. Thus, Layer 2 MPLS services effectively extend services such as Ethernet or frame relay across an IP WAN. To the network operator, Layer 2 MPLS has the distinct advantage of being able to handle a csutomers multi-protocol requirements.
The different types of MPLS
The version of MPLS that’s generally used to encapsulate connection-oriented frame relay and ATM services is called pseudo Wire Edge to Edge Emulation (PWE3). PWE3 defines point-to-point tunnels across the MPLS backbone, and thus works well for circuit-oriented networking protocols. PWE3 can also be used to support connectionless LAN protocols, but it’s not the preferred solution.
For connectionless protocols (primarily Ethernet) there’s a different specification, called virtual private LAN service (VPLS). VPLS addresses some of the specific challenges with extending Ethernet across the metropolitan area or WAN, most notably scalability and availability. Another emerging spec is the ITU’s transport-MPLS (T-MPLS), which is designed to simplify deployment of Ethernet services. It’s worth noting that MPLS isn’t the only game in town when it comes to Ethernet services, though. Several vendors are promoting an alternative approach called Provider Backbone Transport, or PBT, for metropolitan area Ethernet.
PBT is based on using existing IEEE 802.1 VLAN tags to deliver Ethernet services across a provider network. PBT competes head-to-head with T-MPLS, and the jury’s still out on which one will gain the most traction.
The illustration below shows two different types of MPLS service options, namely a Routed Layer 3 option for IP only or the Switched Layer 2 VPLS Option for any protocol.
In the next Fast Cloud article, the Fast Cloud Group will compare and contrast a Layer 3 MPLS VPN (based on RFC 2547) and a Layer 2 VPLS VPN.