spring                                                      R. Geib, Ed.
Internet-Draft                                          Deutsche Telekom
Intended status: Informational                          October 17, 2013
Expires: April 20, 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 scalbale 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  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  A topology aware MPLS path monitoring system  . . . . . . . . . 4
   3.  Applying SR to monitor LDP paths  . . . . . . . . . . . . . . . 5
   4.  PMS monitoring of different Segment ID types  . . . . . . . . . 6
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 6
     7.1.  Normative References  . . . . . . . . . . . . . . . . . . . 6
     7.2.  Informative References  . . . . . . . . . . . . . . . . . . 7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 7

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1.  Introduction

   Paths looping packets through a network are not desireable for
   network and user data routing.  The ability to execute arbitrary path
   combinations within a domain 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 never
   leaving data plane.  This 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.

   The design of such a monitoring system should ensure that OAM packets
   never leave the domain they are supposed to monitor.  Topology and
   network state awareness are useful, careful address-selection may be
   another one.

   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.

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

   IPv6 related ECMP path detection and execution for OAM purposes is
   less powerful than that offered by MPLS OAM.  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

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   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.

2.  A topology aware MPLS path monitoring system

   A MPLS path monitoring system (PMS) which is able to learn all
   labeled paths of a domain is able to build a measurement packet which
   executes an arbitrary chain of paths.  Such a monitoring system is
   aware of the MPLS topology.  The task is to check 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: connected LSRs path to LER i.

   o  Next Label: LER i's path to LER j.

   o  Next Label or address: Data plane measurement destination at LER j
      (this could be a label or an IP address)

   Note that the label stack could as well address MPLS node after MPLS
   node passed by the measurtement packet on it's path from PMS to the
   packets destination or any address stack between this maximum and the
   above minimum address information.  Further, the destination could be
   the PSM itself.  This is shown in figure.

                   +---+     +----+     +-----+
                   |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 a global Node-Segment ID
   label space (meaning the value of a label never changes during a
   label swap).  Let's assign the following Node SIDs to the nodes of

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   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.

3.  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

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   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.

4.  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.

5.  IANA Considerations

   This memo includes no request to IANA.

6.  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
   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.

7.  References

7.1.  Normative References

   [RFC4379]  Kompella, K. and G. Swallow, "Detecting Multi-Protocol
              Label Switched (MPLS) Data Plane Failures", RFC 4379,

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              February 2006.

   [min_ref]  authSurName, authInitials., "Minimal Reference", 2006.

7.2.  Informative References

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

Author's Address

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

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