PMS/Head-end based MPLS Ping and Traceroute in Inter-domain SR Networks
draft-ninan-mpls-spring-inter-domain-oam-02

Routing area                                                    S. Hegde
Internet-Draft                                                  K. Arora
Intended status: Standards Track                           M. Srivastava
Expires: October 18, 2021                          Juniper Networks Inc.
                                                                S. Ninan
                                                  Individual Contributor
                                                                N. Kumar
                                                     Cisco Systems, Inc.
                                                          April 16, 2021

PMS/Head-end based MPLS Ping and Traceroute in Inter-domain SR Networks
              draft-ninan-mpls-spring-inter-domain-oam-02

Abstract

   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.  A network may
   consist of multiple IGP domains or multiple ASes under the control of
   same organization.  It is useful to have the LSP Ping and traceroute
   procedures when an SR end-to-end path spans across multiple ASes or
   domains.  This document describes mechanisms to facilitae LSP ping
   and traceroute in inter-AS/inter-domain SR networks in an efficient
   manner with simple OAM protocol extension which uses dataplane
   forwarding alone for sending Echo-Reply.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

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   This Internet-Draft will expire on October 18, 2021.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Inter domain networks with multiple IGPs  . . . . . . . . . .   4
   3.  Return Path TLV . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Segment sub-TLV . . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Type 1: SID only, in the form of MPLS Label . . . . . . .   5
     4.2.  Type 3: IPv4 Node Address with optional SID for SR-MPLS .   7
     4.3.  Type 4: IPv6 Node Address with optional SID for SR MPLS .   8
     4.4.  Segment Flags . . . . . . . . . . . . . . . . . . . . . .   9
   5.  SRv6 Dataplane  . . . . . . . . . . . . . . . . . . . . . . .  10
   6.  Detailed Procedures . . . . . . . . . . . . . . . . . . . . .  10
     6.1.  Sending an Echo-Request . . . . . . . . . . . . . . . . .  10
     6.2.  Receiving an Echo-Request . . . . . . . . . . . . . . . .  10
     6.3.  Sending an Echo-Reply . . . . . . . . . . . . . . . . . .  10
   7.  Detailed Example  . . . . . . . . . . . . . . . . . . . . . .  11
     7.1.  Procedures for Segment Routing LSP ping . . . . . . . . .  11
     7.2.  Procedures for Segment Routing LSP Traceroute . . . . . .  12
   8.  Building Reverse Path Segment List TLV dynamically  . . . . .  12
     8.1.  The procedures to build the reverse path  . . . . . . . .  13
     8.2.  Details with example  . . . . . . . . . . . . . . . . . .  13
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   11. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  14
   12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  14
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  14
     13.2.  Informative References . . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

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1.  Introduction

                       +----------------+
                       | Controller/PMS |
                       +----------------+

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

                         ASBR2-------ASBR3
                        /             \
                       /               \
    PE1------P1------P2                 P3------P4------PE4
                       \               /
                        \             /
                         ASBR1-------ASBR4

                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].

   [RFC8660] 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 an inter-AS network scenario
   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 consisting of 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.

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

   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
   Ping/Traceroute.

   [RFC7743] describes a Echo-relay based solution based on advertising
   a new Relay Node Address Stack TLV containing stack of Echo-relay ip
   addresses.  That mechanism requires the return ping packet to reach
   the control plane on every relay node.

   This document describes a new mechanism which is efficient and simple
   and can be easily deployed in SR networks.  This mechanism uses
   Return path TLV [RFC7110] to convey the reverse path.  Three new sub-
   TLVs for Return path TLV are defined, that faciliate encoding segment
   routing label stack.  The TLV can either be derived by a smart
   application or controller which has a full topology view.  This
   document also proposes mechanisms to derive the Return path
   dynamically during traceroute procedures.

2.  Inter domain networks with multiple IGPs

    |-Domain 1|-------Domain 2-----|--Domain 3-|

    PE1------ABR1--------P--------ABR2------PE4
     \        / \                  /\        /
      --------   -----------------   -------
       BGP-LU         BGP-LU          BGP-LU

            Figure 2: Inter-domain networks with multiple IGPs

   When the network consists of large number of nodes, the nodes are
   seggregated into multiple IGP domains.  The connectivity to the
   remote PEs can be achieved using BGP-LU [RFC3107] or by stacking the
   labels for each domain as described in [RFC8604].  It is useful to
   support mpls ping and traceroute mechanisms for these networks.  The

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   procedures described in this document for constructing Return path
   TLV and its use in echo-reply is equally applicable to networks
   consisting of multiple IGP domains that use BGP-LU or label stacking.

3.  Return Path TLV

   Segment Routing networks statically assign the labels to nodes and
   PMS/Head-end may know the entire database.  The reverse path can be
   built from PMS/Head-end by stacking segments for the reverse path.
   Return path TLV as defined in [RFC7110] is used to carry the return
   path.  While using the procedures described in this document, the
   reply mode MUST be set to 5 and Return Path TLV MUST be included in
   the Echo-Request message.  The procedures decribed in [RFC7110] are
   applicable for constructing the Return Path TLV.  This document
   define three new sub-TLVs to encode the Segment Routing path

   The type of segment that the head-end chooses to send in the Return
   Path TLV is governed by local policy.

   Some networks may consist of pure IPV4 domains and Pure IPv6 domains.
   Handling end-to-end MPLS OAM for such networks is out of scope for
   this document.  It is recommended to use dual stack in such cases and
   use end-to-end IPv6 addresses for MPLS ping and trace route
   procedures.

4.  Segment sub-TLV

   [I-D.ietf-spring-segment-routing-policy] defines various types of
   segments.  The segments applicable to this documents have been re-
   defined here.  One or more segment sub-TLV can be included in the
   Return Path TLV.  The segment sub-TLVs included in a Return Path TLV
   MAY be of different types.

   Below types of segment sub-TLVs are applicable for the Reverse Path
   Segment List TLV.

   Type 1: SID only, in the form of MPLS Label

   Type 3: IPv4 Node Address with optional SID

   Type 4: IPv6 Node Address with optional SID for SR MPLS

4.1.  Type 1: SID only, in the form of MPLS Label

   The Type-1 Segment Sub-TLV encodes a single SID in the form of an
   MPLS label.  The format is as follows:

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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                      |   Length                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   Flags       |   RESERVED                                    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Label                        | TC  |S|       TTL     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 3: Type 1 Segment sub-TLV

   where:

   Type: TBD1(to be assigned by IANA from the registry "Sub-TLV Target
   FEC stack TLV").

   Length is 8.

   Flags: 1 octet of flags as defined in Section Section 4.4.

   RESERVED: 3 octets of reserved bits.  SHOULD be unset on transmission
   and MUST be ignored on receipt.

   Label: 20 bits of label value.

   TC: 3 bits of traffic class

   S: 1 bit of bottom-of-stack.

   TTL: 1 octet of TTL.

   The following applies to the Type-1 Segment sub-TLV:

   The S bit SHOULD be zero upon transmission, and MUST be ignored upon
   reception.

   If the originator wants the receiver to choose the TC value, it sets
   the TC field to zero.

   If the originator wants the receiver to choose the TTL value, it sets
   the TTL field to 255.

   If the originator wants to recommend a value for these fields, it
   puts those values in the TC and/or TTL fields.

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   The receiver MAY override the originator's values for these fields.
   This would be determined by local policy at the receiver.  One
   possible policy would be to override the fields only if the fields
   have the default values specified above.

4.2.  Type 3: IPv4 Node Address with optional SID for SR-MPLS

   The Type-3 Segment Sub-TLV encodes an IPv4 node address, SR Algorithm
   and an optional SID in the form of an MPLS label.  The format is as
   follows:

       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                      |   Length                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   Flags       |  RESERVED                   | SR Algorithm    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                 IPv4 Node Address (4 octets)                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                SID (optional, 4 octets)                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 4: Type 3 Segment sub-TLV

   where:

   Type: TBD3(to be assigned by IANA from the registry "Sub-TLV Target
   FEC stack TLV").

   Length is 8 or 12.

   Flags: 1 octet of flags as defined in Section Section 4.4.

   SR Algorithm: 1 octet specifying SR Algorithm as described in section
   3.1.1 in [RFC8402], when A-Flag as defined in Section Section 4.4is
   present.  SR Algorithm is used by the receiver to derive the Label.
   When A-Flag is not encoded, this field SHOULD be unset on
   transmission and MUST be ignored on receipt.

   RESERVED: 2 octets of reserved bits.  SHOULD be unset on transmission
   and MUST be ignored on receipt.

   IPv4 Node Address: a 4 octet IPv4 address representing a node.

   SID: 4 octet MPLS label.

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   The following applies to the Type-3 Segment sub-TLV:

   The IPv4 Node Address MUST be present.

   The SID is optional and specifies a 4 octet MPLS SID containing
   label, TC, S and TTL as defined in Section Section 4.1.

   If length is 8, then only the IPv4 Node Address is present.

   If length is 12, then the IPv4 Node Address and the MPLS SID are
   present.

4.3.  Type 4: IPv6 Node Address with optional SID for SR MPLS

   The Type-4 Segment Sub-TLV encodes an IPv6 node address, SR Algorithm
   and an optional SID in the form of an MPLS label.  The format is as
   follows:

       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                      |   Length                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   Flags       |       RESERVED                | SR Algorithm  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      //                IPv6 Node Address (16 octets)                //
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                SID (optional, 4 octets)                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 5: Type 4 Segment sub-TLV

   where:

   Type: TBD4(to be assigned by IANA from the registry "Sub-TLV Target
   FEC stack TLV").

   Length is 20 or 24.

   Flags: 1 octet of flags as defined in Section Section 4.4.

   SR Algorithm: 1 octet specifying SR Algorithm as described in section
   3.1.1 in [RFC8402], when A-Flag as defined in Section Section 4.4 is
   present.  SR Algorithm is used by the receiver to derive the label.
   When A-Flag is not encoded, this field SHOULD be unset on
   transmission and MUST be ignored on receipt.

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   RESERVED: 2 octets of reserved bits.  SHOULD be unset on transmission
   and MUST be ignored on receipt.

   IPv6 Node Address: a 16 octet IPv6 address representing a node.

   SID: 4 octet MPLS label.

   The following applies to the Type-4 Segment sub-TLV:

   The IPv6 Node Address MUST be present.

   The SID is optional and specifies a 4 octet MPLS SID containing
   label, TC, S and TTL as defined in Section Section 4.1 .

   If length is 20, then only the IPv6 Node Address is present.

   If length is 24, then the IPv6 Node Address and the MPLS SID are
   present.

4.4.  Segment Flags

   The Segment Types described above MAY contain following flags in the
   "Flags" field (codes to be assigned by IANA from the registry "Return
   path sub-TLV Flags" )

       0 1 2 3 4 5 6 7
      +-+-+-+-+-+-+-+-+
      | |A|           |
      +-+-+-+-+-+-+-+-+

                              Figure 6: Flags

   where:

   A-Flag: This flag indicates the presence of SR Algorithm id in the
   "SR Algorithm" field applicable to various Segment Types.

   Unused bits in the Flag octet SHOULD be set to zero upon transmission
   and MUST be ignored upon receipt.

   The following applies to the Segment Flags:

   A-Flag is applicable to Segment Types 3, 4.  If A-Flag appears with
   any other Segment Type, it MUST be ignored.

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5.  SRv6 Dataplane

   SRv6 dataplane is not in the scope of this document and will be
   addressed in a separate document.

6.  Detailed Procedures

6.1.  Sending an Echo-Request

   In the inter-AS scenario when there is no reverse path connectivity,
   LSP ping initiator MUST add a Return Path TLV in the Echo-request
   message.  The reverse Segment List MUST correspond to the return path
   from the egress.  The Return Path TLV must contain the Segment
   Routing Path in the reverse direction encoded as an ordered list of
   Segments.  The first Segment MUST correspond to the top Segment in
   MPLS header that the responder MUST use while sending the Echo-reply.

6.2.  Receiving an Echo-Request

   As described in [RFC7110], when Reply mode is set to 5 (Reply via
   Specified Path), The Echo-Request MUST contain the Return path TLV.
   Absence of Reply path TLV is treated as malformed Echo-Request.When a
   Return Path TLV is received, and the responder that supports
   processing it, it MUST use the Segments in Return Path 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 Return Path TLV is used to construct the MPLS
   packet to send out.

6.3.  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 constructed from the first Segment
   from the Return Path TLV.  The remaining labels MUST follow the order
   from the Return Path TLV.  The responder MAY check the reachability
   of the top label in its own LFIB before sending the Echo-Reply.  In
   certain scenarios the head-end may choose to send Type 2/Type 3
   segments consisting of IPV4 address and SID.  In such cases that node
   sending the echo-reply MUST derive the MPLS labels from SIDs based on
   SRGB and encode the echo-reply with MPLS labels.

   The return code MUST be set as described in [RFC7110].  The Return
   Path TLV MUST be included in Echo-Reply indicating the specified
   return path that the echo reply message is required to follow.

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7.  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 have 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
   Segment List stack as below.  N-P1, N-ASBR1, EPE-ASBR1-ASBR4, N-PE4
   for 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.

7.1.  Procedures for Segment Routing LSP ping

   To perform LSP ping procedure on an SR-Path from PE1 to PE4
   consisting of label stacks [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 Return Path from PE4 to PE1 in the MPLS Echo-request using
   multiple Segments in "Return Path TLV" as defined above.  An example
   return path Segment List for PE1 to PE4 for LSP ping is [N-ASBR4,
   EPE-ASBR4-ASBR1, N-PE1].  An implementation may also build a Return
   Path 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 Return Path
   for this case could be [N-ASBR4, EPE-ASBR4-ASBR1].

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   On receiving MPLS Echo-request PE4 first validates FEC in the Echo-
   request.  PE4 then builds label stack to send the response from PE4
   to PE1 by copying the labels from "Return Path TLV".  PE4 builds the
   Echo-reply packet with the MPLS label stack constructed and imposes
   MPLS headers on top of Echo-reply packet and sends out the packet
   towards PE1.  This Segment List stack can successfully steer reply
   back to Head-end node(PE1).

7.2.  Procedures for Segment Routing LSP Traceroute

   As described in the procedures for LSP ping, the reverse Segment List
   may be sent from head-end in which case the LSP Traceroute procedures
   are similar to LSP ping.  The head-end constructs the Return Path TLV
   and the egress node uses the Return Path 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 Return Path segment list
   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 reverse path available from the
   nodes ASBR4 to PE1.  During the Traceroute procedure, when PE1 has to
   visit ASBR4, it builds return Path TLV and includes label to the
   border-node which has the route to, PE1.  In this example the Return
   Path TLV will contain [EPE-ASBR4-ASBR1].  Further down the traceroute
   procedure when P3 or P4 node is being visited, PE1 build the Return
   Path TLV containing [N-ASBR4, EPE-ASBR4-ASBR1].  The Echo-reply will
   be an MPLS packet with this label stack and will be forwarded to PE1.

8.  Building Reverse Path Segment List TLV dynamically

   In some cases, the head-end may not have complete visibility of
   Inter-AS topology.  In such cases, it can rely on downstream routers
   to build the reverse path for mpls traceroute procedures.  For this
   purpose, new return code is defined which implies the Return Path TLV
   in the Echo-reply corresponds to the return path to be used in next
   Echo-Request.

   Value         Meaning
   ------        ----------------------
   0x0006        Use Return Path TLV in Echo-Reply for next Echo-Request.

                           Figure 7: Return Code

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8.1.  The procedures to build the reverse path

   When an ASBR receives an echo-request from another AS, and ASBR is
   configured to build the return path dynamically, ASBR MUST build a
   Return Path TLV that should be used in next Echo-request and add it
   in echo-reply.  ASBR MUST locally decide the outgoing interface for
   the echo-reply packet.  Generally, remote ASBR will choose interface
   on which the incoming OAM packet was receieved to send the echo-reply
   out.  Return Path TLV is built by adding two segment sub TLVs.  The
   top segment sub TLV consists of the ASBR's Node SID and second
   segment consists of the EPE SID in the reverse direction to reach the
   AS from which the OAM packet was received.The type of segment chosen
   to build Return Path TLV is implementation dependent.  In cases where
   the AS is configured with different SRGBs, the Node SID of the ASBR
   should be represented using type 3 segment so that all the nodes
   inside the AS can correctly translate the Node-SID to a label.

   Irrespective of which type of segment is included in the Return Path
   TLV, the responder of echo-request always translates the Return Path
   TLV to a label stack and builds MPLS header for the the echo-reply
   packet.  This procedure can be applied to an end-to-end path
   consisting of multiple ASes.  Each ASBR at the border adds its Node-
   SID and EPE-SID on top of existing segments in the Return Path TLV.

8.2.  Details with example

   Let us consider a traffic engineered path built from PE1 to PE4 with
   a label stack as below.  N-P1, N-ASBR1, EPE-ASBR1-ASBR4, N-PE4 for
   the following procedures.  This traceroute doesn't need any changes
   to the Return Path TLV till it leaves AS1.  The same Return Path TLV
   that is received is included in the Echo-Reply.  But this traceroute
   requires changes to Return Path TLV once it starts probing AS2
   routers.  ASBR4 should form this Return Path TLV using its own Node
   SID(N-ASBR4) and EPE SID (EPE-ASRB4-ASBR1) labels and set the return
   code to 6.  Then PE1 should use this Return Path TLV in subsequent
   echo-requests.  In this example, when the subsequent echo-request
   reaches P3, it should use this Return Path TLV for sending the echo-
   reply.  The same Return Path TLV is enough for any router in AS2 to
   send the reply.  Because the first label(N-ASBR4) can direct echo-
   reply to ASBR4 and second one (EPE-ASBR4-ASBR1) to direct echo-reply
   to AS1.  Once echo reply reaches AS1, normal IP forwarding helps it
   to reach PE1 or the head-end.

9.  Security Considerations

   TBD

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10.  IANA Considerations

   Sub-TLVs for TLV Types 1, 16, and 21

      SID only in the form of mpls label : TBD (Range 32768-65535)

      IPv4 Node Address with optional SID for SR-MPLS : TBD (Range
      32768-65535)

      IPv6 Node Address with optional SID for SR-MPLS : TBD (Range
      32768-65535)

11.  Contributors

   1.Carlos Pignataro

   Cisco Systems, Inc.

   cpignata@cisco.com

   2.  Zafar Ali

   Cisco Systems, Inc.

   zali@cisco.com

12.  Acknowledgments

   Thanks to Bruno Decreane for suggesting use of generic Segment sub-
   TLV.  Thanks to Adrian Farrel for careful review and comments.
   Thanks to Mach Chen for suggesting to use Return Path TLV.

13.  References

13.1.  Normative References

   [I-D.ietf-idr-segment-routing-te-policy]
              Previdi, S., Filsfils, C., Talaulikar, K., Mattes, P.,
              Rosen, E., Jain, D., and S. Lin, "Advertising Segment
              Routing Policies in BGP", draft-ietf-idr-segment-routing-
              te-policy-11 (work in progress), November 2020.

   [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.

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   [RFC7110]  Chen, M., Cao, W., Ning, S., Jounay, F., and S. Delord,
              "Return Path Specified Label Switched Path (LSP) Ping",
              RFC 7110, DOI 10.17487/RFC7110, January 2014,
              <https://www.rfc-editor.org/info/rfc7110>.

   [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>.

   [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>.

13.2.  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.

   [I-D.ietf-spring-segment-routing-policy]
              Filsfils, C., Talaulikar, K., Voyer, D., Bogdanov, A., and
              P. Mattes, "Segment Routing Policy Architecture", draft-
              ietf-spring-segment-routing-policy-09 (work in progress),
              November 2020.

   [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>.

   [RFC3107]  Rekhter, Y. and E. Rosen, "Carrying Label Information in
              BGP-4", RFC 3107, DOI 10.17487/RFC3107, May 2001,
              <https://www.rfc-editor.org/info/rfc3107>.

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   [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>.

   [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>.

   [RFC8660]  Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing with the MPLS Data Plane", RFC 8660,
              DOI 10.17487/RFC8660, December 2019,
              <https://www.rfc-editor.org/info/rfc8660>.

Authors' Addresses

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

   Email: shraddha@juniper.net

   Kapil Arora
   Juniper Networks Inc.

   Email: kapilaro@juniper.net

   Mukul Srivastava
   Juniper Networks Inc.

   Email: msri@juniper.net

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   Samson Ninan
   Individual Contributor

   Email: samson.cse@gmail.com

   Nagendra Kumar
   Cisco Systems, Inc.

   Email: naikumar@cisco.com

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