Traffic Engineering Database dissemination for Hierarchical PCE scenarios

In scenarios with multiple domains in both MPLS-TE and GMPLS networks, the hierarchical Path Computation Element (H-PCE) Architecture, defined in , allows to obtain the optimum end-to-end path. The architecture exploits a hierarchical relation among domains. defines the architecture and requirements for the end-to-end path computation across domains. The solution draft for the H-PCE is focused on the PCEP protocol extensions to support such H-PCE procedures, including negotiation of capabilities and errors. However, neither the architecture nor the solution draft specify which mechanism must to be used to build and populate the parent PCE (pPCE) Traffic Engineering Database (TED). The H-PCE architecture documents define the minimum content needed in the traffic engineering database required to compute paths. The information required by parent TEDB are identified in and further elaborated in . For instance, and suggest that BGP-LS could be used as a "northbound" TE advertisement. This means that a PCE does not need to listen IGP in its domain, but its TED is populated by messages received (for example) from a Route Reflector. extends BGP-LS to disseminate traffic engineering information. The parameters considered are: delay, packet loss and bandwidth. This document highlights the applicability of BGP-LS to the dissemination of domain topology within the H-PCE architecture. In particular, it describes how can BGP-LS be used to send the inter-domain connectivity. It also shows how can OSPF-TE and ISIS-TE updates be mapped into BGP-LS. Note that this document is not intended to define new protocol extensions, it is an informational document and where required it highlights where existing mechanisms and protocols may be applied.

The pPCE maintains a domain topology map of the child domains and their interconnectivity. This map does not include any visibility into the child domains. Where inter-domain connectivity is provided by TE links, the capabilities of those links may also be known to the pPCE. The pPCE maintains a TED for the parent domain, the nodes in the parent domain are abstractions of the cPCE domains (connected by real or virtual TE links), but the pPCE domain may also include real nodes and links. The procedure and protocol mechanism for disseminating and construction of the pPCE TED may be provided using a number of mechanisms, including manually configuring the necessary information or automated using a separate instance of a routing protocol to advertise the domain interconnectivity. Since inter-domain TE links can be advertised by the IGPs operating in the child domains, this information could then be exported to the parent PCE either by the child PCEs or using north-bound export mechanisms.

The information that would be exchanged includes: Identifier of advertising child PCE. Identifier of PCE's domain. Identifier of the link. TE properties of the link (metrics, bandwidth). Other properties of the link (technology-specific). Identifier of link endpoints. Identifier of adjacent domain.

A variety of methods exist to provide are different alternatives so the parent PCE can get the topological information from the child PCEs (cPCEs): Statically configure all inter-domain link and topology information. Membership of an IGP instance. The necessary topological information could be disseminated by joining the IGP instance of each child PCE domain. However, by doing so, it would break the domain confidentiality principles and is subject to scalability issues. PCEP Notification Messages. Another solution is to send the interconnection information between domains using PCEP Notifications (see section 4.8.4 of ). One approach, followed in research work, is embedding in PCEP Notifications the Inter-AS OSPF-TE Link State Advertisements (LSA) to send the Inter-Domain Link information from child PCEs to the parent PCE and to send reachability information (list of end-points in each domain). However, it is argued that the utilization of PCEP to disseminate topology is beyond scope of the protocol. Separate IGP instance. points out that in models such as ASON it is possible to consider a separate instance of an IGP running within the parent domain where the participating protocol speakers are the nodes directly present in that domain and the PCEs (parent and child PCEs). Use north-bound distribution of TE information. The North-Bound Distribution of Link-State and TE Information using BGP has been recently propose in the IEFT . This approach is known as BGP-LS and defines a mechanism by which links state and traffic engineering information can be collected from networks and exported to external elements using the BGP routing protocol. By using BGP-LS as northbound distribution mechanism, there would be a BGP speaker in each domains that sends the necessary information to a BGP speaker in the parent domain. This architecture is further elaborated in this document.

As mentioned in PCE has to retrieve Traffic Engineering (TE) information to carry out its path computation. This is required not only for intra-domain information, which can be got using IGP (like OSPF-TE or ISIS-TE), but also for inter-domain information in the Hierarchical PCE (H-PCE) architecture. shows an example of a H-PCE architecture. In this example, there is a parent PCE and three child PCEs, and they are organized in multiple domains. The parent PCE does not have information of the whole network, but is only aware of the connectivity among the domains and provides coordination to the child PCEs. shows which is the visibility that parent PCE has from the network according to the definition in . Thanks to this topological information, when there is a request to a child PCE with the destination in another domain, this path request is sent to the parent PCE, which selects a set of candidate domain paths and sends requests to the child PCEs responsible for these domains. Then, the parent PCE selects the best solution and it is transmitted to the source PCE.

Thanks to the dissemination of inter-domain adjacency information from each cPCE to the pPCE, the pPCE can have a view of reachability between the domains. The H-PCE architecture with BGP-LS is shown in . Each domain has a cPCE that is able to compute paths in the domain. This child PCE has access to a domain TED, which is built using IGP information. In each domain, a BGP speaker has access to such domain TED and acts as BGP-LS Route Reflector to provide network topology to the pPCE. Next to the pPCE, there is a BGP speaker that maintains a BGP session with each of the BGP speakers in the domains to receive the topology and build the parent TED. A policy can be applied to the BGP-LS speakers to decide which information is sent to its peer speaker. The minimum amount of information that needs to be exchanged is the inter-domain connectivity, including the details of the Traffic Engineering Inter-domain Links . With this information, the parent PCE is able to have access to a domain topology map and its connectivity. Additionally, the BGP-LS speaker can be configured to send some intra-domain information for virtual or candidate paths with some TE information. In this case, the parent PCE has access to an extended database, with visibility of both intra-domain and inter-domain information and can compute the sequence of domains with better accuracy. BGP-LS extends the BGP Update messages to advertise link-state topology thanks to new BGP Network Layer Reachability Information (NLRI). The Link State information is sent in two BGP attributes, the MP_REACH (defined in ) and a LINK_STATE attribute (defined in ). To describe the inter domain links, in the MP_REACH attribute, a Link NLRI can be used with the local node descriptors the address of the source, and in the remote descriptors, the address of the destination of the link. The Link Descriptors field has a TLV (Link Local/Remote Identifiers), which carries the prefix of the Unnumbered or Numbered Interface. In case of the message informs about an intra-domain link, the standard traffic engineering information is included in the LINK_STATE attribute.

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TBD Supporting BGP-4 BGP Speakers Graceful Restart SRLGs Multiprotocol extensions

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