Multiprotocol Label Switching

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"Mpls" redirects here; it is also a common abbreviation for the city of Minneapolis.

In computer networking and telecommunications, MultiProtocol Label Switching (MPLS) is a data-carrying mechanism which emulates some properties of a circuit-switched network over a packet-switched network. MPLS operates at an OSI Model layer that is generally considered to lie between traditional definitions of Layer 2 (data link layer) and Layer 3 (network layer), and thus is often referred to as a "Layer 2.5" protocol. It was designed to provide a unified data-carrying service for both circuit-based clients and packet-switching clients which provide a datagram service model. It can be used to carry many different kinds of traffic, including IP packets, as well as native ATM, SONET, and Ethernet frames.

This is a basic introduction. To obtain more detail on MPLS see External Links.

Contents

[edit] Background

A number of different technologies were previously deployed with essentially identical goals, such as frame relay and ATM. MPLS is now replacing these technologies in the marketplace, mostly because it is better aligned with current and future technology needs.

In particular, MPLS dispenses with the cell-switching and signalling-protocol baggage of ATM. MPLS recognizes that small ATM cells are not needed in the core of modern networks, since modern optical networks (as of 2001) are so fast (at 10 Gbit/s and well beyond) that even full-length 1500 byte packets do not incur significant real-time queuing delays (the need to reduce such delays, to support voice traffic, having been the motivation for the cell nature of ATM).

At the same time, it attempts to preserve the traffic engineering and out-of-band control that made frame relay and ATM attractive for deploying large scale networks.

MPLS was originally proposed by a group of engineers from Cisco Systems, Inc.; it was called "Tag Switching" when it was a Cisco proprietary proposal, and was renamed "Label Switching" when it was handed over to the IETF for open standardization.

One original motivation was to allow the creation of simple high-speed switches, since for a significant length of time it was impossible to forward IP packets entirely in hardware. However, advances in VLSI have made such devices possible. The systemic advantages of MPLS, such as the ability to support multiple service models, do traffic management, etc, remain.

[edit] How MPLS works

MPLS works by prepending packets with an MPLS header, containing one or more 'labels'. This is called a label stack.

MPLS packet

Each label stack entry contains four fields:

  • a 20-bit label value.
  • a 3-bit field for QoS priority.
  • a 1-bit bottom of stack flag. If this is set, it signifies the current label is the last in the stack.
  • an 8-bit TTL (time to live) field.

These MPLS labeled packets are forwarded (switched is the correct term) after a Label Lookup/Switch instead of a lookup into the IP table. Label Lookup and Label Switching may be faster than usual RIB lookup because it can take place directly into fabric and not CPU.

The exit points of an MPLS network are called Label Edge Routers (LER). Routers that are performing routing based only on Label Switching are called Label Switch Routers (LSR). Remember that a LER is not usually the one that is popping the label. For more information see Penultimate Hop Popping.

Devices that function as ingress and/or egress routers are often called PE (Provider Edge) routers. Devices that function only as transit routers are similarly called P (Provider) routers. The job of a P router is significantly easier than that of a PE router, so they can be less complex and may be more dependable because of this.

When an unlabeled packet enters the ingress router and needs to be passed on to an MPLS tunnel, the router first determines the forwarding equivalence class the packet should be in, and then inserts one (or more) labels in the packet's newly created MPLS header. The packet is then passed on to the next hop router for this tunnel.

When a labeled packet is received by an MPLS router, the topmost label is examined. Based on the contents of the label a swap, push (impose) or pop (dispose) operation can be performed on the packet's label stack. Routers can have prebuilt lookup tables that tell them which kind of operation to do based on the topmost label of the incoming packet so they can process the packet very quickly. In a swap operation the label is swapped with a new label, and the packet is forwarded along the path associated with the new label.

In a push operation a new label is pushed on top of the existing label, effectively "encapsulating" the packet in another layer of MPLS. This allows the hierarchical routing of MPLS packets. Notably, this is used by MPLS VPNs.

In a pop operation the label is removed from the packet, which may reveal an inner label below. This process is called "decapsulation". If the popped label was the last on the label stack, the packet "leaves" the MPLS tunnel. This is usually done by the egress router, but see PHP below.

During these operations, the contents of the packet below the MPLS Label stack are not examined. Indeed transit routers typically need only to examine the topmost label on the stack. The forwarding of the packet is done based on the contents of the labels, which allows "protocol independent packet forwarding" that does not need to look at a protocol-dependent routing table and avoids the expensive IP longest prefix match at each hop.

At the egress router, when the last label has been popped, only the payload remains. This can be an IP packet, or any of a number of other kinds of payload packet. The egress router must therefore have routing information for the packet's payload, since it must forward it without the help of label lookup tables. An MPLS transit router has no such requirement.

In some special cases, the last label can also be popped off at the penultimate hop (the hop before the egress router). This is called Penultimate Hop Popping (PHP). This may be interesting in cases where the egress router has lots of packets leaving MPLS tunnels, and thus spends inordinate amounts of CPU time on this. By using PHP, transit routers connected directly to this egress router effectively offload it, by popping the last label themselves.

MPLS can make use of existing ATM network infrastructure, as its labelled flows can be mapped to ATM virtual circuit identifiers, and vice-versa.

[edit] Installing and removing MPLS paths

There are two standardized protocols for managing MPLS paths: CR-LDP (Constraint-based Routing Label Distribution Protocol) and RSVP-TE, an extension of the RSVP protocol for traffic engineering.

An MPLS header does not identify the type of data carried inside the MPLS path; if one wants to carry two different types of traffic between two routers, or ask for different treatment from the core routers for each type, one has to establish one MPLS path for each type of traffic.

[edit] Comparison of MPLS versus IP

MPLS cannot be compared to IP as a separate entity because it works in conjunction with IP and IP's IGP routing protocols. MPLS gives IP networks simple traffic engineering, the ability to transport Layer3 (IP) VPNs with overlapping address spaces, and support for Layer2 pseudo wires (with Any Transport Over Mpls - ATOM - see Martini draft). Routers with programmable CPUs and without TCAM/CAM or another method for fast lookups may also see a limited increase in performance.

MPLS relies on IGP routing protocols to construct its label forwarding table, and the scope of any IGP is usually restricted to a single carrier for stability and policy reasons. As there is still no standard for carrier-carrier MPLS it is not possible to have the same MPLS service (Layer2 or Layer3 VPN) covering more than one operator.

[edit] MPLS Local Protection

Main article: MPLS local protection

In the event of a network element failure when recovery mechanisms are employed at the IP layer, restoration may take several seconds which is unacceptable for real-time applications (such as VoIP)[1] [2][3]. In contrast, MPLS local protection meets the requirements of real-time applications with recovery times comparable to those of SONET rings (up to 50ms).[1][3][4]

[edit] Comparison of MPLS versus ATM

While the underlying protocols and technologies are different, both MPLS and ATM provide a connection-oriented service for transporting data across computer networks. In both technologies connections are signaled between endpoints, connection state is maintained at each node in the path and encapsulation techniques are used to carry data across the connection. Excluding differences in the signaling protocols (RSVP/LDP for MPLS and PNNI for ATM) there still remain significant differences in the behavior of the technologies.

The most significant difference is in the transport and encapsulation methods. MPLS is able to work with variable length packets while ATM transports fixed-length (53 byte) cells. Packets must be segmented, transported and re-assembled over an ATM network using an adaption layer, which adds significant complexity and overhead to the data stream. MPLS, on the other hand, simply adds a label to the head of each packet and transmits it on the network.

Differences exist, as well, in the nature of the connections. An MPLS connection (LSP) is uni-directional - allowing data to flow in only one direction between two endpoints. Establishing two-way communications between endpoints requires a pair of LSPs to be established. Because 2 LSPs are required for connectivity, data flowing in the forward direction may use a different path from data flowing in the reverse direction. ATM point-to-point connections (Virtual Circuits), on the other hand, are bi-directional, allowing data to flow in both directions over the same path (bi-directional are only svc ATM connections; pvc ATM connections are uni-directional).

Both ATM and MPLS support tunnelling of connections inside connections. MPLS uses label stacking to accomplish this while ATM uses Virtual Paths. MPLS can stack multiple labels to form tunnels within tunnels. The ATM Virtual Path Indicator (VPI) and Virtual Circuit Indicator (VCI) are both carried together in the cell header, limiting ATM to a single level of tunnelling.

The biggest single advantage that MPLS has over ATM is that it was designed from the start to be complimentary to IP. Modern routers are able to support both MPLS and IP natively across a common interface allowing network operators great flexibility in network design and operation. ATM's incompatibilities with IP require complex adaptation making it largely unsuitable in today's predominantly IP networks.

[edit] MPLS deployment

MPLS is currently in use in large "IP Only" networks, and is standardized by IETF in RFC 3031.

In practice, MPLS is mainly used to forward IP datagrams and Ethernet traffic. Major applications of MPLS are Telecommunications traffic engineering and MPLS VPN.

[edit] Competitors to MPLS

MPLS can exist currently only in an IPv4 environment (IPv4 routing protocols), and provides limited additional functionality. The major goal of MPLS development - the increase of routing speed - is no longer relevant because of the usage of ASIC, TCAM and CAM based switching. Therefore the major usage of MPLS is to implement limited traffic engineering and Layer3/Layer2 “service provider type” VPNs over existing IPv4 networks. The only competitors to MPLS are technologies like L2TPv3 that also provide services such as service provider Layer2 and Layer3 VPNs.

IEEE 1355 is a completely unrelated technology that does something similar in hardware.

[edit] References

  1. ^ a b Aslam et al. (2005-02-02). "NPP: A Facility Based Computation Framework for Restoration Routing Using Aggregate Link Usage Information". QoS-IP 2005 : quality of service in multiservice IP network. Retrieved on 2006-10-27.
  2. ^ Raza et al.. "Online routing of bandwidth guaranteed paths with local restoration using optimized aggregate usage information". IEEE-ICC 2005. Retrieved on 2006-10-27.
  3. ^ a b Li Li et al.. "Routing bandwidth guaranteed paths with local restoration in label switched networks". Retrieved on 2006-10-27.
  4. ^ Kodialam et al.. "Dynamic Routing of Locally Restorable Bandwidth Guaranteed Tunnels using Aggregated Link Usage Information". IEEE Infocom. pp. 376–385. 2001. Retrieved on 2006-10-27.

[edit] Books

  • "Deploying IP and MPLS QoS for Multiservice Networks: Theory and Practice" by John Evans, Clarence Filsfils (Morgan Kaufmann, 2007, ISBN 0-123-70549-5)

[edit] External links

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