spring R. Geib, Ed. Internet-Draft Deutsche Telekom Intended status: Informational C. Filsfils Expires: August 9, 2014 Cisco Systems, Inc. February 5, 2014 Use case for a scalable and topology aware MPLS data plane monitoring system draft-geib-segment-routing-oam-usecase-00 Abstract This document describes features and a use case of a path monitoring system. Segment based routing enables a scalable and simple method to monitor data plane liveliness of the complete set of paths belonging to a single domain. Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. 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Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as Geib & Filsfils Expires August 9, 2014 [Page 1] Internet-Draft Abbreviated Title February 2014 described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. A topology aware MPLS path monitoring system . . . . . . . . . 4 3. SR based OAM use case illustration . . . . . . . . . . . . . . 5 3.1. Use-case 1 - LSP dataplane liveliness measurement . . . . . 5 3.2. Use-case 2 - Monitoring a remote bundle . . . . . . . . . . 7 3.3. Use-Case 3 - Fault localization . . . . . . . . . . . . . . 7 4. Applying SR to monitor LDP paths . . . . . . . . . . . . . . . 8 5. PMS monitoring of different Segment ID types . . . . . . . . . 8 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8 7. Security Considerations . . . . . . . . . . . . . . . . . . . . 8 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9 8.1. Normative References . . . . . . . . . . . . . . . . . . . 9 8.2. Informative References . . . . . . . . . . . . . . . . . . 9 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9 Geib & Filsfils Expires August 9, 2014 [Page 2] Internet-Draft Abbreviated Title February 2014 1. Introduction It is essential for a network operator to monitor all the forwarding paths observed by the transported user packets. The monitoring flow must be forwarded in dataplane in a similar way as user packets. Problem localization is required. This document describes a solution to this problem statement and illustrates it with use-cases. The solution is described for a single IGP MPLS domain. The solution applies to monitoring of LDP LSP's as well as to monitoring of Segment Routed LSP's. Segment Routing simplifies the solution by the use of IGP-based signalled segments as specified by [ID.sr-isis]. This document adopts the terminology and framework described in [ID.sr-archi]. It further adopts the editorial simplification explained in section 1.2 of the segment routing use-cases [ID.sr-use]. The proposed solution offers several benefits for network monitoring. A single monitoring device is able to monitor the complete set of a domains forwarding paths with OAM packets that never leave data plane. Faults can be localized: o by IGP LSA analysis. o by correlation between different probes. o by MPLS traceroute and adapted ping messages. The proposed solution requires topology awareness as well as a suitable security architecture. Topology awareness is an essential part of link state IGPs. Adding MPLS topology awareness to an IGP speaking device hence enables a simple and scaleable data plane monitoring mechanism. MPLS OAM offers flexible features to recognise an execute data paths of an MPLS domain. By utilsing the ECMP related tool set of RFC 4379 [RFC4379], a segment based routing LSP monitoring system may: o easily detect ECMP functionality and properties of paths at data level. o construct monitoring packets executing desired paths also if ECMP is present. Geib & Filsfils Expires August 9, 2014 [Page 3] Internet-Draft Abbreviated Title February 2014 o limit the MPLS label stack of an OAM packet to a minmum of 3 labels. MPLS OAM supports detection and execution of ECMP paths quite smart. This document is foscused on MPLS path monitoring. The MPLS path monitoring system described by this document can be realised with pre-Segment based Routing (SR) technology. Making monitoring system aware of a domains complete MPLS topolfrom utilising stale MPLS label information, IGP must be monitored and MPLS topology must be timely aligned with IGP topology. Obviously, enhancing IGPs to exchange of MPLS topology information significantly simplifies and stabilises such an MPLS path monitoring system. In addition to IGP extensions, also RFC 4379 may have to be extended to support detection of SR routed paths. Note that the MPLS path monitoring system may be a specialised system residing at a single interface of the domain to be monitored. As long as measurement packets return to this or another well specified interface, the MPLS monitoring system is the single entity pushing monitoring packet label stacks. Concerns about router label stack pushing capabilities don't apply in this case. First drafts discussing requirements, extensions of RFC4379 and possible solutions to allow SR usage as described by this document are at hand, see [ID.sr-4379ext] and [ID.sr-oam_detect]. 2. A topology aware MPLS path monitoring system An MPLS path monitoring system (PMS) which is able to learn the IGP LSDB (including the SID's) is able to build a measurement packet which executes any arbitrary chain of paths. Such a monitoring system is topology aware (all related IP adresses, MPLS SIDs and labels). Let us describe how the PMS can check the liveliness of the MPLS transport path between LER i and LER j. The PMS may do so by sending packets carrying the following minimum address infomation: o Top Label: a path from PMS to LER i This is expressed as Node SID of LER i. o Next Label: the path that needs to be monitored from LER i to LER j. If this path is a single physical interface (or a bundle of connected interfaces), it can be expressed by the related AdjSID. Geib & Filsfils Expires August 9, 2014 [Page 4] Internet-Draft Abbreviated Title February 2014 If the shortest path from LER i to LER j is supposed to be monitored, the Node-SID (LER j) can be used. Another option is to insert a list of segments expressing the desired path (hop by hop as an extreme case). If LER i pushes a stack of Labels based on a SR policy decision and this stack of LSPs is to be monitored, the PMS needs an interface to collect the information enabling it to address this SR created path. o Next Label or address: the path back to the PMS. Likely, no further segment/label is required here. Indeed, once the packet reaches LER j, the 'steering' part of the solution is done and the probe just needs to return to the PMS. This is best achieved by popping the MPLS stack and revealing a probe packet with PMS as destination address (note that in this case, the source and destination addresses could be the same). In this case, a no SID/ label may be assigned to the PMS (if it is a host/server residing in an IP subnet outside the MPLS domain). Note: if the PMS is an IP host not connected to the MPLS domain, the PMS can send its probe with the list of SIDs/Labels onto a suitable tunnel provding an MPLS access to a router which is part of the monitored MPLS domain. 3. SR based OAM use case illustration 3.1. Use-case 1 - LSP dataplane liveliness measurement +---+ +----+ +-----+ |PMS| |LSR1|-----|LER i| +---+ +----+ +-----+ | / \ / | / \__/ +-----+/ /| |LER m| / | +-----+\ / \ \ / \ \+----+ +-----+ |LSR2|-----|LER j| +----+ +-----+ Example of a PMS based LSP dataplane liveness measurement Figure 1 For the sake of simplicity, let's assume that all the nodes are configured with the same SRGB [ID.sr-archi]. as described by section Geib & Filsfils Expires August 9, 2014 [Page 5] Internet-Draft Abbreviated Title February 2014 1.2 of [ID.sr-use]. Let's assign the following Node SIDs to the nodes of the figure: PMS = 10, LER i = 20, LER j = 30. The aim is to check liveliness of the path LER i to LER j. The PMS does this by creating a measurement packet with the following label stack (top to bottom): 20 - 30 - 10. LER m forwards the packet received from the PMS to LSR1. Assuming Pen-ultimate Hop Popping to be deployed, LSR1 pops the top label and forwards the packet to LER i. There the top label has a value 30 and LER i forwards it to LER j. This will be done transmitting the packet via LSR1 or LSR2. The LSR will again pop the top label. LER j will forward the packet now carrying the top label 10 to the PMS (and it will pass a LSR and LER m). A few observations on the example: o The path PMS to LER i must be stable and it must be detectable. o If ECMP is deployed, it may be desired to measure along both possible paths, a packet may use between LER i and LER j. This may be done by using MPLS OAM coded measurement packets with suitable IP destination addresses. o The path LER j to PMS to must be stable and it must be detectable. To ensure reliable results, the PMS should be aware of any changes in IGP or MPLS topology. Determining a path to be executed prior to a measurement may also be done by setting up a label including all node SIDs along that path (if LER1 has Node SID 40 in the example and it should be passed between LER i and LER j, the label stack is 20 - 40 - 30 - 10). Obviously, the PMS is able to check and monitor data plane liveliness of all LSPs in the domain. The PMS may be a router, but could also be dedicated monitoring system. If measurement system reliability is an issue, more than a single PMS may be connected to the MPLS domain. Monitoring an MPLS domain by a PMS based on SR offers the option of monitoring complete MPLS domains with little effort and very excellent scaleability. Geib & Filsfils Expires August 9, 2014 [Page 6] Internet-Draft Abbreviated Title February 2014 3.2. Use-case 2 - Monitoring a remote bundle +---+ _ +--+ +-------+ | | { } | |---991---L1---662---| | |PMS|--{ }-|R1|---992---L2---663---|R2 (72)| | | {_} | |---993---L3---664---| | +---+ +--+ +-------+ SR based probing of all the links of a remote bundle Figure 2 R1 adresses Lx by the Adjacency SID 99x, while R2 adresses Lx by the Adjacency SID 66(x+1). In the above figure, the PMS needs to assess the dataplane availability of all the links within a remote bundle connected to routers R1 and R2. The monitoring system retrieves the SID/Label information from the IGP LSDB and appends the following segment list/label stack: {72, 662, 992, 664} on its IP probe (whose source and destination addresses are the address of the PMS). MS sends the probe to its connected router. If the connected router is not SR compliant, a tunneling technique can be used to tunnel the probe and its MPLS stack to the first SR router. The MPLS/SR domain then forwards the probe to R2 (72 is the Node SID of R2). R2 forwards the probe to R1 over link L1 (Adjacency SID 662). R1 forwards the probe to R2 over link L2 (Adjacency SID 992). R2 forwards the probe to R1 over link L3 (Adjacency SID 664). R1 then forwards the IP probe to PMS as per classic IP forwarding. 3.3. Use-Case 3 - Fault localization In the previous example, a uni-directional fault on the middle link from R1 to R2 would be localized by sending the following two probes with respective segment lists: o 72, 662, 992, 664 o 72, 663, 992, 664 The first probe would fail while the second would succeed. Correlation of the measurements reveals that the only difference is Geib & Filsfils Expires August 9, 2014 [Page 7] Internet-Draft Abbreviated Title February 2014 using the Adjacency SID 662 of the middle link from R1 to R2 in the non successful measurement. Assuming the second probe has been routed correctly, the fault must have been occurring in R2 which didn't forward the packet to the interface identified by its Adjacency SID 662. 4. Applying SR to monitor LDP paths A SR based PMS connected to a MPLS domain consisting of LER and LSR supporting SR and LDP in parrallel in all nodes may use SR paths to transmit packets to and from start and end points of LDP paths to be monitored. In the above example, the label stack top to bottom may be as follows, when sent by the PMS: o Top: SR based Node-SID of LER i at LER m. o Next: LDP label identifying the path to LER j at LER i. o Bottom: SR based Node-SID identifying the path to the PMS at LER j While the mixed operation shown here still requires the PMS to be aware of the LER LDP-MPLS topology, the PMS may learn the SR MPLS topology by IGP and use this information. 5. PMS monitoring of different Segment ID types MPLS SR topology awareness should allow the SID to monitor liveliness of most types of SIDs (this may not be recommendable if a SID identifies an inter domain interface). To match control plane information with data palne information, RFC4379 should be enhaced to allow collection of data relevant to check all relevant types of Segment IDs. 6. IANA Considerations This memo includes no request to IANA. 7. Security Considerations As mentioned in the introduction, a PMS monitoring packet should never leave the domain where it originated. It therefore should never use stale MPLS or IGP routing information. Further, asigning different label ranges for different purposes may be useful. A well Geib & Filsfils Expires August 9, 2014 [Page 8] Internet-Draft Abbreviated Title February 2014 known global service level range may be excluded for utilisation within PMS measurement packets. These ideas shoulddn't start a discussion. They rather should point out, that such a discussion is required when SR based OAM mechanisms like a SR are standardised. 8. References 8.1. Normative References [RFC4379] Kompella, K. and G. Swallow, "Detecting Multi-Protocol Label Switched (MPLS) Data Plane Failures", RFC 4379, February 2006. 8.2. Informative References [ID.sr-4379ext] IETF, "Label Switched Path (LSP) Ping/Trace for Segment Routing Networks Using MPLS Dataplane", IETF, http:// datatracker.ietf.org/doc/draft-kumar-mpls-spring-lsp- ping/, 2013. [ID.sr-archi] IETF, "Segment Routing Architecture", IETF, https:// datatracker.ietf.org/doc/ draft-filsfils-rtgwg-segment-routing/, 2013. [ID.sr-isis] IETF, "IS-IS Extensions for Segment Routing", IETF, http: //datatracker.ietf.org/doc/ draft-previdi-isis-segment-routing-extensions/, 2013. [ID.sr-oam_detect] IETF, "Detecting Multi-Protocol Label Switching (MPLS) Data Plane Failures in Source Routed LSPs", IETF, http:/ /datatracker.ietf.org/doc/draft-kini-spring-mpls-lsp- ping/, 2013. [ID.sr-use] IETF, "Segment Routing Use Cases", IETF, http:// datatracker.ietf.org/doc/ draft-filsfils-rtgwg-segment-routing-use-cases/, 2013. Geib & Filsfils Expires August 9, 2014 [Page 9] Internet-Draft Abbreviated Title February 2014 Authors' Addresses Ruediger Geib (editor) Deutsche Telekom Heinrich Hertz Str. 3-7 Darmstadt, 64295 Germany Phone: +49 6151 5812747 Email: Ruediger.Geib@telekom.de Clarence Filsfils Cisco Systems, Inc. Brussels, Belgium Phone: Email: cfilsfil@cisco.com Geib & Filsfils Expires August 9, 2014 [Page 10]