An MPLS SR OAM option reducing the number of end-to-end path validations

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Author Ruediger Geib 
Last updated 2021-10-25
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Internet Engineering Task Force                             R. Geib, Ed.
Internet-Draft                                          Deutsche Telekom
Intended status: Best Current Practice                   25 October 2021
Expires: 28 April 2022

An MPLS SR OAM option reducing the number of end-to-end path validations


   MPLS traceroute implementations validate dataplane connectivity and
   isolate faults by sending messages along every end-to-end Label
   Switched Path (LSP) combination between a source and a destination
   node.  This requires a growing number of path validations in networks
   with a high number of equal cost paths between origin and
   destination.  Segment Routing (SR) introduces MPLS topology awareness
   combined with Source Routing.  By this combination, SR can be used to
   implement an MPLS traceroute option lowering the total number of LSP
   validations as compared to commodity MPLS traceroute.

Status of This Memo

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
   2.  MPLS OAM adding MPLS SR mechanisms  . . . . . . . . . . . . .   5
     2.1.  Operation in an SR MPLS domain applying only IP-header
           based ECMP  . . . . . . . . . . . . . . . . . . . . . . .   6
     2.2.  Operation in an SR MPLS domain additionally using incoming
           interface information for ECMP  . . . . . . . . . . . . .   7
   3.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   Commodity MPLS isn't topology aware and it doesn't support
   standardized source routing methods.  It is reasonable to validate
   connectivity and locate faults of MPLS LSPs by detecting and testing
   all existing LSP combinations between a source and a destination
   node.  The source node originates all MPLS echo requests and
   evaluates all MPLS echo replies.  Operational MPLS OAM
   implementations were present, when SR MPLS entered standardisation.
   They continue to work reliably in many cases.  MPLS domains with a
   high number of equal cost paths between source and destination nodes
   push the detection capabilities of commodity MPLS OAM to the limit.
   So far, modes of MPLS OAM operation adding Segment Routing
   functionality to deal with limitations of commodity MPLS OAM have not
   been published within IETF.

   This draft assumes readers to be aware of MPLS OAM functionality as
   specified by RFC 8029 [RFC8029] and RFC 8287 [RFC8287].  The function
   described in the following works for Shortest Path First Paths or
   Label stacks based on MPLS Node-SID and MPLS Adj-SIDs (if the latter
   are distributed by Interior Gateway Protocols).

   Networks supporting a high number of equivalent cost paths between
   source and destination nodes require a high number of completed MPLS
   path validations.  Consider a network with Multiple equal cost paths,
   as shown in figure 1.

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            /        \
           8          12
          /            \
       /                  \
      4  numbers indicate  4
     /   parallel links     \
   RS                        RD
     \    symmetric to      /
      4...upper network ...4

                                  Figure 1

   Figure 1: Multiple equal cost path example network.

   The total number of MPLS LSP combinations between nodes RS and RD is
   multiplicative by the number of (equal cost, so to say) links per
   hop.  That results in a maximum of 4096=2*4*(8*12+8*4)*4 path
   combinations which a commodity MPLS may try to validate.  Assume RS
   to start an MPLS traceroute to RD containing a Multipath Data Sub-TLV
   requesting Multipath information for 32 IP-addresses.  By Equal Cost
   Multipath routing (ECMP, [RFC2991]) traffic of likely 16 of these IP-
   addresses is forwarded via R110 as next hop (the other 16 addresses
   are assumed to be forwarded along the symmetric and equal cost paths
   in the lower half, which are omitted in the figure for brevity).
   R110 can be expected to respond by an MPLS echo reply indicating
   prefixes to address each of the 4 equal cost (sub-)paths between RS
   and R110.

   R110 is able to forward traffic addressed by these 16 IP addresses
   via 16 equal cost paths.  There's a fairly high probability that this
   will not be possible, as some of R110's availble paths to forward
   traffic to RD will receive traffic of two or even three MPLS echo
   request destination IP addresses resuulting in an MPLS Echo request
   being sent from RS to R110 and ahead, while other equal cost paths of
   R110 receive no traffic at all.  The MPLS Echo Reply returned to RS
   will indicate that.  A commodity solution is, to start an additional
   MPLS traceroute from RS with another 32 destination IP-addresses.
   This may help to then enable forwarding of MPLS Echo requests along
   all of R110's paths to RD via R120 and R121, respectively.  With bad
   luck, R110 will forward only 14 or 15 addresses via R120.  R120
   forwards MPLS Echo requests along 12 equal cost paths to RD.  Then
   again, there's a fair chance that more destination IP-addresses are
   required to forward at least one MPLS echo request along all of R120
   equal cost paths to RD.  Per each new set of destination IP-
   addresses, the MPLS Echo-Request / Reply dialogue must be completed
   starting from RS to at least all routers along the path to R120.

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   In the example, roughly only a fourth of the addresses whose
   forwarding is validated starting from node RS will be routed via
   R120.  ECMP load balancing "filters away" 75% of MPLS Echo requests
   carrying the destination IP-addresses whose forwarding is to be
   determined.  If MPLS Echo requests carrying a full set of 32
   destination IP-addresses were reaching R120, the probability of being
   unable to forward at least one MPLS Echo request to each outgoing
   interface (or path, respectively) at R110 destined to node RD was
   rather small.

   The reason for completing all MPLS Echo Request / Reply dialogues
   along the path between RS and R120 is figuring out, which destination
   IP-addresses are routed from R110 to R120 to be available at the
   latter for forwarding traffic along paths to RD which can't be
   addressed otherwise.  RFC 8029 section 4.1 'Dealing with Equal-Cost
   Multipath (ECMP)' concludes, that 'full coverage may not be possible'

   Segment Routing (SR) allows node RS to forward MPLS Echo Request
   packets with up to, e.g., 32 IP addresses to every node which RS
   detects on a path to node RD.  Doing so reduces the number of local
   router path options to be checked to no more than the sum of the
   interfaces belonging to one of the ECMP routes between nodes RS and
   RD.  In the case of the example network above, this sum is
   2*(4+8+8+12+4+4)=80 different local router interfaces of routers RS,
   R110, R120, R121 and R130.  That means, that around 2% of the
   messages and MPLS Label Switched Path checks required with commodity
   MPLS traceroute implementations are sufficient to validate all local
   forwarding options for paths from RS to RD (note that the calculation
   isn't exact, it rather indicates the order of magnitude).  The
   commodity MPLS OAM implementations are neither broken nor not
   working.  SR allows an additional router local MPLS OAM method to
   validate high numbers of ECMP routes reliably and fast.  That method
   proposed here reduces the number of MPLS Echo-Request / -Reply
   dialogues to be stored and completed at the origing of the path

   The functions specified by this document do not require changes in
   the MPLS OAM protocol as specified by [RFC8029] and [RFC8287].

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

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2.  MPLS OAM adding MPLS SR mechanisms

   MPLS Segment Routing (SR) provides each node of an MPLS SR domain
   with this domain's MPLS Node-SID topology [RFC8402].  The SR source
   routing feature allows to forward packets to each individual node
   within a SR domain.  Combining topology awareness and source routing
   allows complete validation of all local router shortest path
   forwarding options from an RS node to an RD node in a domain
   supporting ECMP.

   Suppose SR to be deployed in the case of the example network and
   digits following the letter "R" to indicate the corresponding Node-
   SIDs.  Assume "mixed operation" of commodity MPLS OAM and the option
   applying SR.  RS starts a commodity MPLS Echo request to R110.  After
   having received an MPLS Echo reply from R110 indicating local paths
   of R110 on which none of the packets with the remaing 16 IP addresses
   will be forwarded, RS creates an MPLS Echo Request which transports
   the original 32 IP addresses to R110.  To do so, an additional top-
   Segment is pushed carrying the R110 Node-SID, 110.  The message below
   that additional segment is coded as a standard RFC8287 MPLS Echo
   request.  Two things are special: the TTL of the MPLS header
   containing the Node SID of RD is always set to 1.  Further, a
   seperate sequence number series needs to be started to distinguish
   the starting point of this SR using MPLS OAM sequence.  Coding space
   for MPLS OAM Sender's Handle and Sequence Number offer sufficient
   coding space [RFC8029].  If PHP is active, the R110 Node-SID is
   implicitly present only on the link to a neighboring node.  Still
   packets with all 32 IP-destination addresses are forwarded to R110.
   The chances to address all of the 16 ECMP paths of R110 to RD with
   the originally configured 32 IP-addresses increase.  The same method
   is repeated for R120.  Now the top Segment picked by node RS is the
   Node-SID of R120, again with a separate Sender's Handle and Sequence
   Number combination.  Note, that the MPLS Echo request destined to
   R120 doesn't require execution of MPLS OAM functions in R110.  That
   latter node simply forwards the packet to R120.  Also R120 receives
   32 IP-addresses (which is a significant increase as compared to
   commodity MPLS OAM).

   As a result, the MPLS Echo reply tables maintained by RS likely
   indicate ECMP several forwarding masks of the same IP address range
   (discerned by the starting node receiving the MPLS Echo request with
   top Segment TTL=1).  For every path at an indermediate node, to which
   the latter can't foward an MPLS Echo request due to the limited
   number of available IP-addresses, a suitable SR top segement is added
   for an additional next MPLS Echo request of node RS.  This in the end
   allows to circumvent the IP-address filtering effect caused by ECMP.

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   Being able to forward a "complete" set of IP addresses to any
   interface along an end-to-end path is helpful in locating errors.
   Different MPLS OAM addressing options also offer more possibilities
   to test and unambiguosly locate a faultily sub-path.

2.1.  Operation in an SR MPLS domain applying only IP-header based ECMP

   The basic operation is to transport an MPLS Echo request from the
   sender node sequentially to a next hop identified on any of the paths
   to a destination node.  This is done by applying standard SR
   methodology, which here consists of pushing one additional Node-SID
   on top of the Label-stack to be validated by the sender node.  The
   Node-SID is set to the value of the node, whose forwarding plane
   information is requested by the MPLS Echo request.  This is
   illustrated by figure 2.

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       |Node-SID of the node whose forwarding information is requested |
       |                                                               |
       +                 Sender node MPLS Echo request                 +
       |                                                               |

                                Figure 2

   Figure 2: MPLS OAM Label Stack in the case of IP-header only based

   The added Node-SID is only added to use standard MPLS forwarding.
   The TTL of this added Node-SID set to the default value for traffic
   injected by the sending router.  The MPLS-TC may be set to a value
   ensuring reliable transport up to the node, whose forwarding
   information is requested by the sender node (be aware of MPLS-TC
   treatment of the node popping this added Node-SID in that case).

   The TTL of the top Label of the sender node MPLS Echo request which
   is contained below the added Node-SID initially is set to TTL=1.
   Other TTL values can be picked if LSPs from the intermediate node
   onwards to the destination node of that FEC are desired to be traced
   or pinged by MPLS OAM messages.

   Two modes of operation exist: either applying legacy MPLS OAM and
   adding the described functionality as required or only applying the
   option specified here.  Note that the exact path from the sender node
   to the intermediate node identified by the pushed Node-SID is only

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   known to the node originating and maintaining the MPLS traceroute
   information, if only one path exists between that sender node and an
   intermediate node.

   If the method is added to commodity MPLS OAM functions, the
   originatior IP-address of an MPLS Echo-reply indicating a lack of IP-
   addresses to forward traffic along all ECMP egress interfaces at that
   intermediate node can be used to derive the Node-SID to be pushed by
   the MPLS Echo request sender node.

2.2.  Operation in an SR MPLS domain additionally using incoming
      interface information for ECMP

   This option can only be applied, if the Segment Routing domain's Adj-
   SID topology is known to the node originating MPLS Echo Request
   messages.  Configuring the the Interior Gateway Protocol to
   distribute Adj-SIDs conveniently enables that.  If ECMP is
   additionally using the incoming interface of a packet for path
   selection, an Adj-SID is added between the Node-SID and the MPLS Echo
   request.  As the idea is to determine the incoming interface of the
   node, whose ECMP path choices are requested by MPLS OAM, the
   additionaly pushed Node-SID here is that of the node preceding the
   intermediate node, whose forwarding information is requested.  The
   Adj-SID is chosen to correspond to a specific incoming interface of
   the intermediate node whose forwarding information is requested.  As
   the aim of that test is to ensure that every incoming to outgoing
   interface path choice of the intermediate node can be addressed, the
   topology information required to identify the upstream Adj-SID
   corresponding to an incoming interface of the intermediate node is
   assumed to be present and maintained in the originating node.  This
   additional MPLS to IP topology excerpt information results from prior
   MPLS path validations of the same basic set of MPLS path validations
   between the source node and the destination node (this is to express,
   that no extra measurement effort is caused, as correlation of
   available information is sufficient).  The resulting label stack is
   illustrated by figure 3.

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       |Node-SID of node preceding the node whose fwd info is requested|
       |Adj-SID corresp. to inc-IF of node whose fwd info is requested |
       |                                                               |
       +                 Sender node MPLS Echo request                 +
       |                                                               |

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                                Figure 3

   Figure 3: MPLS OAM Label Stack applying SR features if ECMP is
   additionally based on incoming interfaces.

   In the network example of figure 1, node RS picks the Node-SID of
   R110 and an Adj-SID of R110 corresponding to a particular incoming
   interface of R120, if the latter's ECMP path also depends on the
   incoming interface, by which the MPLS Echo request was received.

   Here, the full set of original IP-addresses can be forwarded
   individually per incoming interface of the router whose MPLS
   forwarding information is requested.  In the example above, it is
   node R120 (not node R110.)  Monitoring incoming interface based ECMP
   results in a higher number of MPLS OAM validations, no matter whether
   commodity MPLS OAM is applied or the option specified here.  The
   overall sum of tests now is determinde by the sum of per node
   incoming * outgoing paths ( or interfaces, respectively).  If the
   method specified here is applied in the case of the example network,
   2*(4*8 + 4*8 + 8*12 + 8*4 + 12*4 + 4*4) = 512 MPLS Echo-Request /
   Response validations are required.  Note that this is still a smaller
   number than the original 4096 path validations in the case of
   comodity MPLS OAM required for a domain applying ECMP based on IP-
   address information only.  Note that the number of required MPLS OAM
   path validations is increasing significantly, if ECMP forwarding is
   in addition based on incoming interfaces and the product of a nodes
   incoming * outgoing interfaces is high.

3.  IANA Considerations

   This memo includes no request to IANA.

4.  Security Considerations

   This document does not introduce new functionality.  It combines
   Segment Routing functions with those of MPLS OAM.  The related
   security sections apply, see [RFC8029] and [RFC8402].

5.  References

5.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

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   [RFC2991]  Thaler, D. and C. Hopps, "Multipath Issues in Unicast and
              Multicast Next-Hop Selection", RFC 2991,
              DOI 10.17487/RFC2991, November 2000,

   [RFC8029]  Kompella, K., Swallow, G., Pignataro, C., Kumar Nainar,
              N., Aldrin, S., and M. Chen, "Detecting Multiprotocol
              Label Switched (MPLS) Data-Plane Failures", RFC 8029,
              DOI 10.17487/RFC8029, March 2017,

   [RFC8287]  Kumar Nainar, N., Pignataro, C., Swallow, G., Akiya, N.,
              Kini, S., and M. Chen, "Label Switched Path (LSP) Ping/
              Traceroute for Segment Routing (SR) IGP-Prefix and IGP-
              Adjacency Segment Identifiers (SIDs) with MPLS Data
              Planes", RFC 8287, DOI 10.17487/RFC8287, December 2017,

   [RFC8402]  Filsfils, C., Previdi, S., Ginsberg, L., Decraene, B.,
              Litkowski, S., and R. Shakir, "Segment Routing
              Architecture", RFC 8402, DOI 10.17487/RFC8402, July 2018,

Author's Address

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

   Phone: +49 6151 5812747

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