PMS/Head-end based MPLS Ping and Traceroute in Inter-AS SR Networks
draft-ninan-spring-mpls-inter-as-oam-00

Internet-Draft                Inter-as-OAM                     July 2019

    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
   Identifiers(SIDs).

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

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

   EPE SIDS : EPE-ASBR2-ASBR3, EPE-ASBR1-ASBR4, EPE-ASBR3-ASBR2 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,
   EPE-ASBR4-ASBR1].

   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

   TBD

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

   [I-D.ietf-spring-segment-routing-central-epe]
              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,
              <https://www.rfc-editor.org/info/rfc8287>.

8.2.  Informative References

   [I-D.ietf-idr-bgpls-segment-routing-epe]
              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.

   [I-D.ietf-mpls-interas-lspping]
              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
              2007.

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   [I-D.ietf-spring-segment-routing-mpls]
              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,
              <https://www.rfc-editor.org/info/rfc2119>.

   [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, <https://www.rfc-editor.org/info/rfc7743>.

   [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,
              <https://www.rfc-editor.org/info/rfc8029>.

   [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, <https://www.rfc-editor.org/info/rfc8402>.

   [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, <https://www.rfc-editor.org/info/rfc8403>.

   [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,
              <https://www.rfc-editor.org/info/rfc8604>.

Authors' Addresses

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

   Email: shraddha@juniper.net

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   Kapil Arora
   Juniper Networks Inc.

   Email: kapilaro@juniper.net

   Samson Ninan
   Juniper Networks Inc.

   Email: samsonn@juniper.net

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