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Versions: 00 01

Routing area                                                    K. Arora
Internet-Draft                                                  S. Hegde
Intended status: Standards Track                   Juniper Networks Inc.
Expires: August 25, 2019                                       S. Aldrin
                                                                  Google
                                                            S. Litkowski
                                                 Orange Business Service
                                                              M. Durrani
                                                                 Equinix
                                                       February 21, 2019


    TTL Procedures for SR-TE Paths in Label Switched Path Traceroute
                               Mechanisms
          draft-arora-mpls-spring-ttl-procedures-srte-paths-01

Abstract

   Segment routing supports the creation of explicit paths using
   adjacency-sids, node-sids, and anycast-sids.  The SR-TE paths are
   built by stacking the labels that represent the nodes and links in
   the explicit path.  A very useful Operations And Maintenance
   requirement is to be able to trace these paths as defined in
   [RFC8029].  This document specifies a uniform mechanism to support
   MPLS traceroute for the SR-TE paths when the nodes in the network are
   following uniform mode or short-pipe mode [RFC3443].

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
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   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 August 25, 2019.

Copyright Notice

   Copyright (c) 2019 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
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Problem with SR-TE Paths  . . . . . . . . . . . . . . . . . .   3
     2.1.  Short Pipe model  . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Uniform Model . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Detailed Solution For TTL procedures for SR-TE paths  . . . .   5
     3.1.  P bit in DDMT TLV . . . . . . . . . . . . . . . . . . . .   5
       3.1.1.  Procedures for a PHP router of the tunnel being
               traced  . . . . . . . . . . . . . . . . . . . . . . .   5
       3.1.2.  Procedures for a egress  router of the tunnel being
               traced  . . . . . . . . . . . . . . . . . . . . . . .   5
       3.1.3.  Procedures for a ingress router of the SR-TE path . .   5
       3.1.4.  Example describing the solution . . . . . . . . . . .   6
     3.2.  Procedures for handling binding-sids  . . . . . . . . . .   7
       3.2.1.  Uniform Model . . . . . . . . . . . . . . . . . . . .   7
       3.2.2.  Shortpipe Model . . . . . . . . . . . . . . . . . . .   8
   4.  Backward Compatibility  . . . . . . . . . . . . . . . . . . .   8
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   The mechanisms to handle TTL procedures for SR-TE paths are described
   in ([RFC8287]).  Section 7.5 of ([RFC8287]) defines the TTL
   manipulation procedures for short pipe model as below.The LSR



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   initiating the traceroute SHOULD start by setting the TTL to 1 for
   the tunnel in the LSP's label stack it wants to start the tracing
   from, the TTL of all outer labels in the stack to the max value, and
   the TTL of all the inner labels in the stack to zero.  However this
   mechanism has issues when the constituent tunnels are penultimate-
   hop-popping(PHP).  This document does not propose any change to
   ([RFC8287]) if the constituent tunnels are ultimate-hop-popping (UHP)
   or Egress LSR advertizes explicit NULL.

   Section 2 describes problems in tracing SR-TE paths and the need for
   a specialized mechanism to trace SR-TE paths.  Section 3 describes
   the solution applied to mpls echo request/response to trace
   adjacency-sids and node-sids trace SR-TE path in uniform model and
   short pipe model.

2.  Problem with SR-TE Paths

   The topology shown in Figure 1. illustrates a example network
   topology with SPRING enabled on each node.

      Node          Node          Node          Node
      sid:1         sid:2         sid:3         sid:4
      +----+   10   +----+   10   +----+   10   +----+
      | R1 |--------| R2 |--------| R3 |--------| R4 |
      +----+        +----+        +----+        +----+

         Label stack:
        +------------+
        |  1003 (top)|
        +------------+
        |  1004      |
        +------------+


              Figure 1: Example topology with SRGB 1000-2000

   Consider an explicit path in the topology in Figure 1 from R1->R4 via
   R1->R2->R3->R4.  The label stack to instantiate this path contains
   two node-sids 1003 and 1004.  The 1003 label will take the packet
   from R1 to R3.  The next label in the stack 1004 will take the packet
   from R3 to the destination R4. consider the mechanism below for the
   TTL procedures specified in RFC 8287 for short pipe model and uniform
   model for PHP LSPs.

   Notation: ((X,Y),(Z,W)) refers to a label stack whose top label stack
   entry has the label corresponding to the node-SID of X, with TTL Y,
   and whose second label stack entry has the label corresponding to the
   node-SID of Z, with TTL W.



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   According to the procedure in Section 7.5 of [RFC8287], the LSP
   traceroute is done as follows in short pipe model and uniform model:

2.1.  Short Pipe model

   Refer the diagram in Figure 1.

   1.  Ingress R1 sends mpls LSP Echo Request with label stack of
   ((1003,1),(1004,0)) to R2.

   2.  Since R2 receives mpls LSP Echo Request with TTL as 1 for outer
   most label, R2's local software processes the Lsp traceroute packet
   and R2 sends an echo reply to R1 with return code as 'transit'.

   3.  R1 receives the LSP Echo Reply from R2, and then sends next LSP
   Echo Request with label stack ((1003,2),(1004,0)).

   4.  R2 forwards packet to R3 as ((1004,0)) (i.e.  R2 being PHP, pops
   the label 1003 and does not propagate TTL)

   5.  R3 receives a packet with TTL=0 at the top of the stack.  Receipt
   of a packet with TTL=0 may cause R3 to drop the packet or rate limit
   it.

   6.  Even if R3's local software processes the packet and validates
   the FEC for 1003 and sends egress code in echo-reply, the next packet
   will have ((1003,255), (1004, 1)) which causes TTL to expire again on
   R3 as the 1003 label is popped at the penultimate.

   RFC 8287 suggests that when R1's LSP Echo Request has reached the
   egress of the outer tunnel, R1 should begin to trace the inner tunnel
   by sending a LSP Echo Request with label stack ((1003,255),(1004,1)).
   However, as explained in step 6, the traceroute procedure does not
   work correctly.

2.2.  Uniform Model

   1.  Ingress R1 sends mpls LSP Echo Request with label stack of
   ((1003,1),(1004,0)) to R2.

   2.  Since R2 receives mpls LSP Echo Request with TTL as 1 for outer
   most label, R2's local software processes the Lsp ping packet and R2
   sends an echo reply to R1 with return code as 'transit'.

   3.  R1 receives the LSP Echo Reply from R2, and then sends next LSP
   Echo Request with label stack ((1003,2),(1004,0)).





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   4.  It is expected that R2 should propogate the TTL of outer label to
   inner label before forwarding the packet to R3.  However most of the
   PFEs implementations generally do not increase a label stack entry's
   TTL when they do TTL propagation.  So when (1003,2) is popped, we
   might still end up with (1004,0) at R3, even if we have TTL
   propagation configured.  Increasing the TTL of a packet is not a good
   practice as it can result in forwarding loops.

   5.  R3 receives a packet with TTL=0 at the top of the stack.  Receipt
   of a packet with TTL=0 will cause R3 to drop the packet or rate limit
   it.

   6.  Even if R3's local software processes the packet and validates
   the FEC for 1003 and sends egress code in echo-reply, the next packet
   will have ((1003,255), (1004, 1)) which causes TTL to expire again on
   R3 as the 1003 label is popped at the penultimate.

   So in either case (uniform model or short pipe model) traceroute may
   not work for SR-TE paths with PHP Lsps.

3.  Detailed Solution For TTL procedures for SR-TE paths

3.1.  P bit in DDMT TLV

   DS flags has 4 unused bits from position '0' to '3'.  This document
   uses bit '3' in DS flags of downstream mapping TLV.

3.1.1.  Procedures for a PHP router of the tunnel being traced

   When a LSR receives an echo request it MUST validate the outermost
   FEC in the echo request.  LSR SHOULD set the 'P' bit in the DS flags
   of downstream mapping TLV if its a PHP router for the outermost FEC.
   Other cases it should work as explained in [RFC8029] and [RFC8287].

3.1.2.  Procedures for a egress router of the tunnel being traced

   When a LSR receives an echo request it MUST validate the outermost
   FEC in the echo request.  Egress cases should work as explained in
   [RFC8029] and [RFC8287].

3.1.3.  Procedures for a ingress router of the SR-TE path

   When an ingress LSR receives an echo response it MUST behave as
   defined below depending on the return code in the echo response.

   1.  When an ingress LSR receives an echo response with return code as
   8 (Label switched at stack-depth), Ingress LSR MUST check if the LSR
   that sent the echo response is PHP for the outermost FEC in the FEC



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   stack.  If the LSR that sent the echo response is PHP for the
   outermost FEC then while sending next echo request Ingress LSR MUST
   increase the TTL value of inner label also (if exists) in addition to
   increasing the TTL value of the tunnel it is tracing.  Ingress LSR
   can detect that LSR that sent the echo response is a PHP router for
   the outermost FEC, either by looking at 'P' bit set in the DS flags
   of downstream mapping TLV or if Ingress LSR has received LABEL '3' in
   the label stack TLV of downstream detailed mapping TLV.  For all
   other cases ingress should work as explained in [RFC8029] and
   [RFC8287].

   2.  When an Ingress LSR receives an echo response with return code as
   3 (Replying router is an egress for the FEC at stack-depth) for the
   outermost FEC and this is not the only FEC in the FEC stack, then
   ingress LSR SHOULD remove the outermost FEC from the FEC stack and
   send the next traceroute request with the same TTL value for all the
   labels in the label stack as the previous echo request.  This will
   ensure the egress of the tunnel is visited twice, once as egress for
   top label and again as a transit for next tunnel.

3.1.4.  Example describing the solution

   This section provides a detailed description of how PHP router helps
   ingress in handling TTL procedures for SR-TE paths.  Below are the
   procedures performed by PHP router and ingress router to perform TTL
   procedure for mpls traceroute for SR-TE paths.  Below solution works
   for both uniform model and short pipe model.

   1.  Ingress R1 sends mpls LSP Echo Request with label stack of
   ((1003,1),(1004,0)) to R2.

   2.  Since R2 receives mpls LSP Echo Request with TTL as 1 for outer
   most label, R2's local software processes the Lsp ping packet.  R2's
   local software validates the outermost FEC and looking at the FEC R2
   knows that its the PHP router for outermost FEC (Node-Sid R3).

   3.  R2 sets a bit in the DS flags in the DDMT TLV in echo response (P
   bit, One of the reserved bits).

   4.  When R1 looks at the echo response from R2 it sees P bit in DDMT
   TLV.

   5.  So R1 increments the TTL value of Node-R3 by 1 (make it 2) and
   TTL value of next element in the label stack also

   6.  R1 should send the next mpls LSP Echo Request with label stack
   ((1003,2),(1004,1)).




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   7.  R2 being PHP pops the outermost label from the label stack and
   forwards the packet to R3 with with label (1004, 1)

   8.  R3 receives mpls LSP Echo Request with TTL as 1 for outer most
   label, R3's local software processes the echo request.

   9.  R3 validates the outermost FEC and sends echo response to R1 with
   return code as the egress for outermost FEC (Node-Sid R3).

   10.  When R1 receives echo response with return code as egress, R1
   should remove outermost FEC (Node-Sid R3) from the FEC stack and send
   the next echo request with the same TTL value as the previous one i.e
   ((1003,2),(1004,1)).

   11.  Since R3 is the PHP router for FEC (Node-Sid R4) in the label
   stack.  R3 should set 'P' bit in the in the DS flags in the DDMT TLV
   in echo response with return code as Transit.

   12.  R1 should send the next mpls LSP Echo Request with label stack
   ((1003,2),(1004,2)) with FEC Node-Sid-R4 .

   13.  R2 pops the first label from the label stack and R3 pops the
   second label from the label stack.

   14.  R4 receives an unlabelled packet with RA bit set in ip options.
   R4 delivers the packet to local software for processing.

   15.  R4's local software validates the ouetmost FEC as 'egress' and
   sends an echo reply with return code as egress.

   17.  R1 receives an echo reply with return code as egress for the
   last FEC in the FEC stack TLV and completes the traceroute.

3.2.  Procedures for handling binding-sids

   Inorder to provide greater scalability, network opacity, and service
   independence, SR architecture [RFC8402] defines a Binding SID (BSID).
   A Binding SID is bound to an SR policy which typically involves a
   list of SIDs.  These Binding SIDs may appear in another SR Policy or
   may be used to steer service traffic from the service origin.  The
   TTL handling mechanisms for MPLS traceroute procedures involving
   Binding SIDs is described below.

3.2.1.  Uniform Model

   When the node advertising the Binding SID is operating in uniform
   mode [RFC3443], it SHOULD send FEC stack change sub-TLV as in sec
   4.5.1 of [RFC8029].  The ingress node SHOULD increment the TTL of



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   Binding SID label at every step until "egress" return code is sent
   for all the new FECs included due to FEC stack change and all the
   Tunnels replaced by the Binding SID are completely traced.  It is
   required that all the label popping nodes involved in these tunnels
   MUST support uniform model and copy the TTL to bottom label when the
   label is popped.

3.2.2.  Shortpipe Model

   When the node advertising the Binding SID is operating in short pipe
   model [RFC3443], it SHOULD not send FEC stack change sub-TLV.  The
   Binding SID is treated as single hop and the nodes internal to the
   Tunnel represented by Binding SID SHOULD NOT be traced.

4.  Backward Compatibility

   The extension proposed in this document is backward compatible with
   procedures described in [RFC8029] and [RFC8287].  If the LSR with the
   proposed solution is the Ingress and all other LSR in the SR tunnel
   are not with the extension, Then no LSR is going to set 'P' bit so
   ingress LSR with new extension will work as per [RFC8029] and
   [RFC8287].If the LSR with the proposed extension is the one of the
   transit router and if its the PHP then it may set 'P' bit based on
   the section 3.  Ingress may not react to the 'P' bit and traceroute
   will continue to work as per [RFC8029] and [RFC8287].

5.  Security Considerations

   TBD

6.  IANA Considerations

   IANA has created and now maintains a registry entitled "DS Flags".
   The registration policy for this registry is Standards Action
   [RFC5226].  IANA has made the following assignments:

   Bit Number Name Reference

   ---------- ------------------------------------------- ---------

   7 N: Treat as a Non-IP Packet [RFC8029]

   6 I: Interface and Label Stack Object Request [RFC8029]

   5 E: ELI/EL push indicator [RFC8012]

   4 L: Label-based load balance indicator [RFC8012]




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   3 P: Penulimate Hop router

   2-0 Unassigned

7.  Acknowledgements

   Thanks to Przemyslaw Krol for careful review and comments.

8.  References

8.1.  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,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <https://www.rfc-editor.org/info/rfc5226>.

   [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>.

   [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>.

8.2.  Informative References

   [RFC3443]  Agarwal, P. and B. Akyol, "Time To Live (TTL) Processing
              in Multi-Protocol Label Switching (MPLS) Networks",
              RFC 3443, DOI 10.17487/RFC3443, January 2003,
              <https://www.rfc-editor.org/info/rfc3443>.





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Authors' Addresses

   Kapil Arora
   Juniper Networks Inc.
   Exora Business Park
   Bangalore, KA  560103
   India

   Email: kapilaro@juniper.net


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

   Email: shraddha@juniper.net


   Sam Aldrin
   Google

   Email: aldrin.ietf@gmail.com


   Stephane Litkowski
   Orange Business Service

   Email: stephane.litkowski@orange.com


   Muhammad Durrani
   Equinix

   Email: mdurrani@equinix.com















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