MPLS Working Group Internet Draft S. Van den Bosch D. Papadimitriou Document: draft-vandenbosch-mpls-fa- Alcatel considerations-00.txt Expires: August 2002 February 2002 Further considerations for Forwarding Adjacency LSPs Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC 2026 [1]. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Abstract Forwarding adjacencies (FA) as described in [2] are a useful tool for improving the scalability of Generalized MPLS (GMPLS) Traffic Engineering (TE). Through the aggregation of TE LSPs this concept enables the creation of a TE-LSP Hierarchy. Forwarding adjacency LSPs (FA-LSP or simply FA) may be advertised as TE link into the same instance of ISIS/OSPF as the one that was used to create the LSP, allowing other LSRs to use FAs as TE-links for their path computation. As such, forwarding adjacency LSP have characteristics of both links and LSPs. This document list a number of topics for further study regarding forwarding adjacencies in an attempt to identify what future work may result in the MPLS WG (or other) from the link/LSP duality. Conventions used in this document The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [3]. Van den Bosch et al. Informational - Expires August 2002 1 Further considerations for FA-LSPs February 2002 Table of Contents Status of this Memo................................................1 Abstract...........................................................1 Conventions used in this document..................................1 1. Introduction...................................................2 2. Topics for further study.......................................2 3. A Step Further.................................................4 Security Considerations............................................4 References.........................................................5 Acknowledgments....................................................5 Author's Addresses.................................................5 Full Copyright Statement...........................................5 1. Introduction In its successful attempt to extend MPLS for non-packet oriented TE- attributes within the scope of an integrated model encompassing several layers, which are controlled using the same routing and signaling protocol instances, GMPLS has been rapidly confronted to mapping of the overlaid transport layers within the control plane. As such, and from the control plane viewpoint a transport plane layer is mapped to an LSP region (see [2]). Following this approach, a TE-link (at the control plane) maps exactly the definition the one proposed in the canonical layered model where a link is a link bundle (using the IETF terminology). Today, the FA concept enables the definition of hierarchical TE-Links, enabling in turn to decrease the number of control plane instances to control N transport layers. While this is in perfect alignment for non-packet LSP regions and boundaries, the same concept(s) can be re-used in the MPLS LSP context but with a major difference. The mapping goes in the opposite direction from the control to the IP/MPLS forwarding plane since FA-LSP in the packet domain are purely abstract concept that would if well tailored, provide additional scalability within a routing plane instance (in particular, an area). In this context, the present document tries to identify and briefly address some of the issues for FA-LSP in a packet-switching (PSC) context that could require further effort from the Sub-IP Area community. 2. Topics for further study This section list some topics, this might be extended in future release of this document. 2.1. Establishment and teardown of FA for PSC interfaces A LSR can have multiple interface Switching Capabilities (SC). This aspect is of particular importance with packet switch capable (PSC) interfaces. These effectively constitute a logical hierarchy that Van den Bosch et al. Informational - Expires August 2002 2 Further considerations for FA-LSPs February 2002 can be exploited through label stacking in order to provide scalable TE in large single area networks. However, combining such an approach, with constraint-based TE- routing would imply the need for dynamic modification of an interface's switching capability in order to promote/demote it in the logical hierarchy (corresponding to the creation/deletion of a layer in the hierarchy). The other way is to advertise multiple PSC capabilities, and so let the network dynamically build-up this hierarchy. In such an eventuality, the arising complexity comes up from the ôdynamic modificationö that would arise from such policy since each Switching Type decision is taken independently by each ingress LSR (when initiating the packet LSP Path/Label Request message). 2.2. Dynamic recomputation of paths for FA-LSPs The path information regarding the FA-LSP is made available to other nodes by means of the PATH TLV. This information may be used by other nodes for their own path computations. They could for instance take it into account when computing a backup path. Then if the FA- LSP changes path, primary and backup path of the client LSPs may no longer be disjoint. Therefore, it may be necessary to foresee the possibility to restrict the re-routability of the FA-LSP and advertise this in the PATH TLV. In this way, ingress LSR that use this information would know whether it is subject to any change at a lower LSP region or not. 2.3. Protection of FA-LSPs Four packet interface switching capabilities (PSC-1 to PSC-4) are defined (see [2]. When FA-LSPs are protected this means that SRLG information needs to propagate to the upper LSP regions for consistency. It is not obvious how the SRLG information and disjointness of FA-LSPs can be treated in an efficient way with a label stack of potentially depth four (PSC1-4). 2.4. Interworking with fast reroute techniques An FA-LSP is at the same time a path to the server layer and a link to the client layer. In this context, link protection techniques such as the ones proposed for fast local restoration may be applicable. More specifically, one could for instance envisage the establishment of FA-LSPs for the intra-area sections of a multi-area LSP. In that case, fast local reroute techniques applied at the FA- LSP level could provide inter-area protection of multi-area LSPs. In addition, this would require a fast failure detection and notification mechanism for the FA-LSP. 2.5. Traffic engineering attributes of FA-LSPs Van den Bosch et al. Informational - Expires August 2002 3 Further considerations for FA-LSPs February 2002 It is not clear how the TE attributes (defined as Sub-TLVs of the Opaque LSA TE-Link TLV), such as link colour and metric, of a FA-LSP should be set. The current specification (MPLS-HIER), for instance, proposes to treat FA-LSP as uncoloured links. This may create ambiguity with local policy decisions. It may also hinder the efficient establishment of LSPs with resource requirements over FA- LSPs. 3. A Step Further As a matter of fact, the FA concept is more than mature to achieve a first set of features, we propose here to enlarge the scope to achieve better scalability in (G)MPLS-TE networks keeping in mind the trade-off between functionality and complexity. Security Considerations The approach outlined in this document poses no new security problems. Van den Bosch et al. Informational - Expires August 2002 4 Further considerations for FA-LSPs February 2002 References 1 S. Bradner, "The Internet Standards Process -- Revision 3", BCP 9, RFC 2026, October 1996. 2 K. Kompella, Y. Rekhter, "LSP Hierarchy with Generalized MPLS TE", work in progress, draft-ietf-mpls-lsp-hierarchy-04.txt 3 S. Bradner, "Key words for use in RFCs to Indicate Requirement Levels," BCP 14, RFC2119. Acknowledgments Author's Addresses Sven Van den Bosch Alcatel Francis Wellesplein 1 Phone: 32-3-240-8103 B-2018 Antwerpen Email: sven.van_den_bosch@alcatel.be Belgium Dimitri Papadimitriou Alcatel Francis Wellesplein 1 Phone: 32-3-240-8491 B-2018 Antwerpen Email: dimitri.papadimitriou@alcatel.be Belgium Full Copyright Statement "Copyright (C) The Internet Society (date). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implmentation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. 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Informational - Expires August 2002 5 Further considerations for FA-LSPs February 2002 NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Van den Bosch et al. Informational - Expires August 2002 6