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.

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Table of Contents

• 1. Introduction
• 2. A topology aware MPLS path monitoring system
• 3. SR based OAM use case illustration
• 3.1. Use-case 1 - LSP dataplane liveliness measurement
• 3.2. Use-case 2 - Monitoring a remote bundle
• 3.3. Use-Case 3 - Fault localization
• 4. Applying SR to monitor LDP paths
• 5. PMS monitoring of different Segment ID types
• 6. IANA Considerations
• 7. Security Considerations
• 8. References
• 8.1. Normative References
• 8.2. Informative References
• Authors' Addresses

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:

• by IGP LSA analysis.
• by correlation between different probes.
• 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:

• easily detect ECMP functionality and properties of paths at data level.
• construct monitoring packets executing desired paths also if ECMP is present.
• 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:

• Top Label: a path from PMS to LER i This is expressed as Node SID of LER i.
• 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. 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.
• 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|
                           +----+     +-----+
           

            +---+    _   +--+                    +-------+
            |   |   { }  |  |---991---L1---662---|       |
            |PMS|--{   }-|R1|---992---L2---663---|R2 (72)|
            |   |   {_}  |  |---993---L3---664---|       |
            +---+        +--+                    +-------+