Generalized Multiprotocol Label Switching: An Overview of Routing and Management Enhancements
Ayan Banerjee, John Drake, Jonathan P. Lang, and Brad Turner, Calient Networks
Kireeti Kompella, Juniper Networks
Yakov Rekhter, Cisco Systems
Over the last few years, IP routing has evolved to include new functionality under the umbrella of multiprotocol label switching (MPLS) [4], and recent work has been done on extending MPLS as a control plane that can be used not merely with routers, but also with legacy equipment (e.g., SONET, ADMs) and newer devices like OXCs [5]. These efforts offer the necessary standardized common control plane, an essential component in the evolution of open and interoperable optical networks. First, a common control plane simplifies operations and management, which reduces the cost of operations. Second, a common control plane provides a wide range of deployment scenarios ranging from overlay to peer, where the overlay model is realized by using just a subset of the functionality provided by the peer model. A common control plane allows the choice of peer or overlay (or a combination of both) to be driven by business considerations, rather than constrained by technology. At the same time, building the common control plane from proven signaling and routing avoids “rein-venting the wheel” for protocol development, thereby minimizing risk while reducing time to market.
Some modifications and additions are required to the MPLS routing and signaling protocols to adapt to the peculiarities of photonic switches. These are being standardized by the Internet Engineering Task Force (IETF) under the umbrella of generalized MPLS, which can be summarized as follows:
A new Link Management Protocol (LMP) designed to address issues related to link management in optical networks using photonic switches [6]
MPLS BACKGROUND
MPLS is based on the following set of ideas:
Forwarding information (label) separate from the content of IP header
A single forwarding paradigm (label swap-ping), multiple routing paradigms
Multiple link-specific realizations of the label swapping forwarding paradigm:
“shim,” virtual connection/path identifier
(VCI/VPI), frequency slot (wavelength),
time slot
The flexibility to form forwarding equivalence classes (FECs)
A forwarding hierarchy via label stacking
The separation of forwarding information from the content of the IP header allows MPLS to be used with devices such as OXCs, whose data plane cannot recognize the IP header. Label switch routers (LSRs) forward data using the label carried by the data. This label, combined with the port on which the data was received, is used to determine the output port and outgoing label for the data. The MPLS control plane operates in terms of the label swapping and forwarding paradigm abstraction. At the same time, the MPLS data plane allows multiple link-specific realizations of this abstraction. For example, a wavelength could be viewed as an implicit label. Finally, the concept of a forwarding hierarchy via label stacking enables interaction with devices that can support only a small label space. This property of MPLS is essential in the context of OXCs and DWDMs since the number of wavelengths (which act as labels) is not very large.
The MPLS framework includes significant applications such as constraint-based routing. Constraint-based routing is a combination of extensions to existing IP link-state routing protocols (e.g., OSPF and IS-IS) with RSVP or CR-LDP as the MPLS control plane, and the Constrained Shortest-Path-First (CSPF) heuristic. The extensions to OSPF and IS-IS allow nodes to exchange information about network topology, resource availability and even policy information. This information is used by the CSPF [10, sec. 7] heuristic to compute paths subject to specified resource and/or policy constraints.
For example, either RSVP-TE [11] or CR-LDP [12] is used to establish the label for-warding state along the routes computed by a CSPF-based algorithm; this creates the LSP. The MPLS data plane is used to forward the data along the established LSPs.
Constraint-based routing is used today for two main purposes: traffic engineering and fast reroute.
With suitable network design, the constraint-based routing of IP/MPLS can replace ATM as the
mechanism for traffic engineering. Likewise, fast reroute offers an alternative to SONET as a
mechanism for protection/restoration. Both traffic engineering and fast reroute are examples of how
enhancements provided by MPLS to IP routing make it possible to bypass ATM and SONET/SDH
by migrating functions provided by these technologies to the IP/MPLS control plane
Paving a path for future evolution of MPLS technologies, as well as GMPLS enhancements, are several emerging synergies between LSRs and photonic switches, and between an LSP and an optical trail. An optical trail is an end-to-end path composed exclusively of photonic elements without optical-electronic conversions. Analogous to switching labels in an LSR, a photonic switch toggles wavelengths from an input to an output port. Establishing an LSP involves configuring each intermediate LSR to map a particular input label and port to an output label and port. Similarly, the process of establishing an optical trail involves configuring each intermediate pho-tonic switch to map a particular input lambda and port to an output lambda and port. As in LSRs, photonic switches need routing protocols like OSPF or IS-IS to exchange link-state topology and other optical resource availability information for path computation. They also need signaling protocols like RSVP and LDP to automate the path establishment process. In the remainder of this article, we discuss the routing enhancements and link management.
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