Routing area                                                    S. Hegde
Internet-Draft                                                  K. Arora
Intended status: Standards Track                                S. Ninan
Expires: January 5, 2020                           Juniper Networks Inc.
                                                            July 4, 2019

  PMS/Head-end based MPLS Ping and Traceroute in Inter-AS SR Networks


   Segment Routing (SR) architecture leverages source routing and
   tunneling paradigms and can be directly applied to the use of a
   Multiprotocol Label Switching (MPLS) data plane.  Segment Routing
   also provides an easy and efficient way to provide inter connectivity
   in a large scale network as described in [RFC8604].  [RFC8287]
   illustrates the problem and defines extensions to perform LSP Ping
   and Traceroute for Segment Routing IGP-Prefix and IGP-Adjacency
   Segment Identifiers (SIDs) with an MPLS data plane.  It is useful to
   have the LSP Ping and traceroute procedures when an SRend-to-end path
   spans across multiple ASes.  This document describes mechanisms to
   facilitae LSP ping and traceroute in inter-AS SR networks in an
   efficient manner with simple OAM protocol extension.

Requirements Language

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

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
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   This Internet-Draft will expire on January 5, 2020.

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Copyright Notice

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   document authors.  All rights reserved.

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   ( in effect on the date of
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Reverse Path label stack TLV  . . . . . . . . . . . . . . . .   4
     2.1.  Reverse Path label stack TLV definition . . . . . . . . .   4
     2.2.  SRv6 Dataplane  . . . . . . . . . . . . . . . . . . . . .   5
   3.  Detailed Procedures . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Sending an Echo-Request . . . . . . . . . . . . . . . . .   5
     3.2.  Receiving an Echo-Request . . . . . . . . . . . . . . . .   6
     3.3.  Sending an Echo-Reply . . . . . . . . . . . . . . . . . .   6
   4.  Detailed Example  . . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  Procedures for Segment Routing LSP ping . . . . . . . . .   7
     4.2.  Procedures for Segment Routing LSP Traceroute . . . . . .   7
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   8
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

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                       | Controller/PMS |

    |---------AS1----------|         |--------AS2----------|

                        /             \
                       /               \
    PE1------P1------P2                 P3------P4------PE4
                       \               /
                        \             /

                Figure 1: Inter-AS Segment Routing topology

   Many network deployments have built their networks consisting of
   multiple Autonomous Systems either for ease of operations or as a
   result of network mergers and acquisitions.  Segment Routing can be
   deployed in such scenarios to provide end to end paths, traversing
   multiple Autonomous systems(AS).  These paths consist of Segment
   Identifiers(SID) of different type as per [RFC8402].

   [I-D.ietf-spring-segment-routing-mpls] specifies the forwarding plane
   behaviour to allow Segment Routing to operate on top of MPLS data
   plane.  [I-D.ietf-spring-segment-routing-central-epe] describes BGP
   peering SIDs, which will help in steering packet from one Autonomous
   system to another.  Using above SR capabilities, paths which span
   across multiple Autonomous systems can be created.

   For example Figure 1 describes a topology consisting of inter-AS
   network consisting of ASes AS1 and AS2.  Both AS1 and AS2 are Segment
   Routing enabled and the EPE links have EPE labels configured and
   advertised via [I-D.ietf-idr-bgpls-segment-routing-epe].  Controller
   or head-end can build end-to-end Traffic-Engineered path Node-SIDs,
   Adjacency-SIDs and EPE-SIDs.  It is advantageous for operations to be
   able to perform LSP ping and traceroute procedures on these inter-AS
   SR paths.  LSP ping/traceroute procedures use ip connectivity for
   Echo-reply to reach the head-end.  In inter-AS networks, ip
   connectivity may not be there from each router in the path.For
   example in Figure 1 P3 and P4 may not have ip connectivity for PE1.

   [RFC8403] describes mechanisms to carry out the mpls ping/traceroute
   from a PMS.  It is possible to build GRE tunnels or static routes to
   each router in the network to get IP connectivity for the return

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   path.  This mechanism is operationally very heavy and requires PMS to
   be capable of building huge number of GRE tunnels, which may not be

   It is not possible to carry out LSP ping and Traceroute functionality
   on these paths to verify basic connectivity and fault isolation using
   existing LSP ping and Traceroute mechanism([RFC8287] and [RFC8029]).
   This is because, there exists no IP connectivity to source address of
   ping packet, which is in a different AS, from the destination of

   [RFC7743] describes a Echo-relay based solution based on advertising
   a new Relay Node Address Stack TLV containing stack of Echo-relay ip
   addresses.  This mechanism requires the return ping packet to reach
   the control plane on every relay node.  This document describes a
   mechanism which is efficient and simple and can be easily deployed in
   SR networks.

2.  Reverse Path label stack TLV

   Segment Routing networks statically assign the labels to nodes and
   PMS/Head-end knows entire database.  The return path can be built
   from PMS/Head-end by stacking labels for the return path.  A new TLV
   "Reverse Path label stack TLV" is defined.  Each TLV contains a list
   of labels which may be a prefix/adjacency/binding SID/EPE SID.  MPLS
   Echo -request should contain this TLV, which defines reverse path to
   reach source from the destination.

   The new Reverse Path label stack TLV is an optional TLV.  This TLV is
   carried in the Echo-Request message.  This optional TLV MAY appear in
   the Echo-request message in any order before or after Target FEC
   Stack TLV.  The Reverse Path label stack TLV is defined as below.
   Each MPLS Echo-request SHOULD contain this TLV in inter-AS cases,
   which will enable remote end to send the reply to source.

2.1.  Reverse Path label stack TLV definition

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    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
   |Type = TBD                     |          Length               |
   |     No. of labels             |          Reseved              |
   |     Label (20 bits)                   |   Reserved            |
   |                              ...                              |

                  Figure 2: Reverse Path label stack TLV

   Type: TBD

   Length: Length of TLV including TLV header

   No.  Of elements in set:

   Ordered set of Labels in the Reverse Path label stack

   Label :

   Represents 20 bit label.  This field should be used to build the
   return packet.  The first label in the label stack represents the top
   most label that should be encoded in the return packet.

2.2.  SRv6 Dataplane

   A future version of this document will address the SRv6 Dataplane.

3.  Detailed Procedures

3.1.  Sending an Echo-Request

   LSP ping initiator MUST add a Return Path Label Stack TLV in the
   Echo-request message.  The return label stack MUST correspond to the
   return path from the egress.  The Reverse Path Label Stack TLV is an
   ordered list of labels.  The first label corresponds to the top label
   that the reponder should use to construct the Echo-reply.

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3.2.  Receiving an Echo-Request

   When a receiver does not understand the Reverse Path Label Stack TLV,
   it SHOULD silently ignore the TLV and proceed with normal processing
   as described in [RFC8029].When a Reverse Path Label Stack TLV is
   received, and the responder supports processing it, it MUST use the
   labels in Reverse Path Label Stack TLV to build the echo-reply.  The
   responder MUST follow the normal FEC validation procedures as
   described in [RFC8029] and [RFC8287] and this document does not
   suggest any change to those procedures.  When the Echo-reply has to
   be sent out the Reverse Path label Stack TLV is used to construct the
   MPLs packet to send out.

3.3.  Sending an Echo-Reply

   The Echo-Reply message is sent as mpls packet with a mpls label
   stack.  The Echo-Reply message MUST be constructed as described in
   the [RFC8029].  An MPLS packet is constructed with Echo-reply in the
   payload.  The top label MUST be the first label from the Reverse Path
   Label Stack TLV.  The remaining labels MUST follow the order from the
   Reverse Path Label Stack.  The responder MAY check the reachability
   of the top label in its own LFIB before sending the Echo-Reply.

4.  Detailed Example

   An example topology is given in Figure 1 . This will be used in below
   sections to explain LSP Ping and Traceroute procedures.  The PMS/
   Head-end has complete view of topology.  PE1, P1, P2, ASBR1 and ASBR2
   are in AS1.  Similarly ASBR3, ASBR4, P3, P4 and PE4 are in AS2.

   AS1 and AS2 are Segment Routing enabled.  IGPs like OSPF/ISIS are
   used to flood SIDs in each Autonomous System.  The ASBR1, ASBR2,
   ASBR3, ASBR4 advertise BGP EPE SIDs for the inter-AS links.  Topology
   of AS1 and AS2 are advertised via BGP-LS to the controller/PMS or
   Head-end node.  The EPE-SIDs are also advertised via BGP-LS as
   described in [I-D.ietf-idr-bgpls-segment-routing-epe]

   The description in the document uses below notations for Segment

   Node SIDs : N-PE1, N-P1, N-ASBR1 etc.

   Adjacency SIDs : Adj-PE1-P1, Adj-P1-P2 etc.


   Let us consider a traffic engineered path built from PE1 to PE4 with
   label stack as below.  N-P1, N-ASBR1, EPE-ASBR1-ASBR4, N-PE4 for

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   following procedures.  This stack may be programmed by controller/PMS
   or Head-end router PE1 may have imported the whole topology
   information from BGP-LS and computed the inter-AS path.

4.1.  Procedures for Segment Routing LSP ping

   To perform LSP ping procedure on an SR-Path from PE1 to PE4
   consisting of label stack [N-P1,N-ASBR1,EPE-ASBR1-ASBR4, N-PE4], The
   remote end(PE4) needs IP connectivity to head end(PE1) for the
   Segment Routing ping to succeed, because Echo-reply needs to travel
   back to PE1 from PE4.  But in typical deployment scenario there will
   be no ip route from PE4 to PE1 as they belong to different ASes.

   PE1 adds Reverse Path from PE4 to PE1 in the MPLS Echo-request using
   multiple labels in "Reverse Path Label Stack TLV" as defined above.
   An example return path label stack for PE1 to PE4 for LSP ping i
   [N-ASBR4, EPE-ASBR4-ASBR1, N-PE1].  An implementation may also build
   a Reverse Path Label stack consisting of labels to reach its own AS.
   Once the label stack is popped-off the Echo-reply message will be
   exposed.The further packet forwarding will be based on ip lookup.  An
   example Reverse Path Label Stack for this case could be [N-ASBR4,

   On receiving MPLS Echo-request PE4 first validates FEC in the Echo-
   request and calculates label stack to send the response from PE4 to
   PE1 using "Return label stack TLV".  PE4 builds the Echo-reply packet
   with the mpls label stack constructed out of Reverse Path Label Stack
   TLV and sends out the Echo-reply to PE1.  This label stack can
   successfully steer reply back to Head-end node(PE1).

4.2.  Procedures for Segment Routing LSP Traceroute

   As described in the procedures for LSP ping, the return label stack
   may be sent from head-end in which case the LSP Traceroute procedures
   are similar to mpls-ping.  The head-end constructs the Reverse Path
   Label Stack TLV and the egress node uses the Reverse Path Label Stack
   to construct the Echo-reply packet header.  Head-end/PMS is aware of
   the return path from every node visited in the network and builds the
   Reverse Path Label Stack for every visited node accordingly.

   For Example:

   For the same traffic engineered path PE1 to PE4 mentioned in above
   sections, let us assume there is no return path available from the
   nodes ASBR2 to PE1.  During the Traceroute procedure, when PE1 has to
   visit ASBR2, it builds Return Path Label Stack TLV and includes label
   to the border-node which has the route to, PE1.  In this example the
   Return Path Label Stack TLV will contain [EPE-ASBR2-ASBR1].  Further

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   down the traceroute procedure when P3 or P4 node is being visited,
   PE1 build the Return Path Label Stack TLV containing [N-ASBR2, EPE-
   ASBR2-ASBR1].  The Echo-reply will be an mpls packet with this label
   stack and will be forwarded to PE1.

5.  Security Considerations


6.  IANA Considerations

   Multiprotocol Label Switching (MPLS) Label Switched Paths (LSPs) Ping
   Parameters TLVs Registry

      Reverse Path label stack TLV : TBD

7.  Acknowledgments

8.  References

8.1.  Normative References

              Filsfils, C., Previdi, S., Dawra, G., Aries, E., and D.
              Afanasiev, "Segment Routing Centralized BGP Egress Peer
              Engineering", draft-ietf-spring-segment-routing-central-
              epe-10 (work in progress), December 2017.

   [RFC8287]  Kumar, N., Ed., Pignataro, C., Ed., 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,

8.2.  Informative References

              Previdi, S., Talaulikar, K., Filsfils, C., Patel, K., Ray,
              S., and J. Dong, "BGP-LS extensions for Segment Routing
              BGP Egress Peer Engineering", draft-ietf-idr-bgpls-
              segment-routing-epe-19 (work in progress), May 2019.

              Nadeau, T. and G. Swallow, "Detecting MPLS Data Plane
              Failures in Inter-AS and inter-provider Scenarios", draft-
              ietf-mpls-interas-lspping-00 (work in progress), March

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              Bashandy, A., Filsfils, C., Previdi, S., Decraene, B.,
              Litkowski, S., and R. Shakir, "Segment Routing with MPLS
              data plane", draft-ietf-spring-segment-routing-mpls-22
              (work in progress), May 2019.

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

   [RFC7743]  Luo, J., Ed., Jin, L., Ed., Nadeau, T., Ed., and G.
              Swallow, Ed., "Relayed Echo Reply Mechanism for Label
              Switched Path (LSP) Ping", RFC 7743, DOI 10.17487/RFC7743,
              January 2016, <>.

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

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

   [RFC8403]  Geib, R., Ed., Filsfils, C., Pignataro, C., Ed., and N.
              Kumar, "A Scalable and Topology-Aware MPLS Data-Plane
              Monitoring System", RFC 8403, DOI 10.17487/RFC8403, July
              2018, <>.

   [RFC8604]  Filsfils, C., Ed., Previdi, S., Dawra, G., Ed.,
              Henderickx, W., and D. Cooper, "Interconnecting Millions
              of Endpoints with Segment Routing", RFC 8604,
              DOI 10.17487/RFC8604, June 2019,

Authors' Addresses

   Shraddha Hegde
   Juniper Networks Inc.
   Exora Business Park
   Bangalore, KA  560103


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   Kapil Arora
   Juniper Networks Inc.


   Samson Ninan
   Juniper Networks Inc.


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