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


   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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   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

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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

   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

   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

   o  construct monitoring packets executing desired paths also if ECMP
      is present.

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   o  limit the MPLS label stack of an OAM packet to a minmum of 3

   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

   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.

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      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

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   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.

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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

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   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

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   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

              IETF, "Label Switched Path (LSP) Ping/Trace for Segment
              Routing Networks Using MPLS Dataplane", IETF,  http://
              ping/, 2013.

              IETF, "Segment Routing Architecture", IETF,  https://
              draft-filsfils-rtgwg-segment-routing/, 2013.

              IETF, "IS-IS Extensions for Segment Routing", IETF,  http:
              draft-previdi-isis-segment-routing-extensions/, 2013.

              IETF, "Detecting Multi-Protocol Label Switching (MPLS)
              Data  Plane Failures in Source Routed LSPs", IETF,  http:/
              ping/, 2013.

              IETF, "Segment Routing Use Cases", IETF,  http://
              draft-filsfils-rtgwg-segment-routing-use-cases/, 2013.

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Authors' Addresses

   Ruediger Geib (editor)
   Deutsche Telekom
   Heinrich Hertz Str. 3-7
   Darmstadt,   64295

   Phone: +49 6151 5812747
   Email: Ruediger.Geib@telekom.de

   Clarence Filsfils
   Cisco Systems, Inc.

   Email: cfilsfil@cisco.com

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