TTL Procedures for SR-TE Paths in Label Switched Path Traceroute Mechanisms
draft-arora-mpls-spring-ttl-procedures-srte-paths-00

Routing area                                                    K. Arora
Internet-Draft                                                  S. Hegde
Intended status: Standards Track                   Juniper Networks Inc.
Expires: April 19, 2019                                 October 16, 2018

    TTL Procedures for SR-TE Paths in Label Switched Path Traceroute
                               Mechanisms
          draft-arora-mpls-spring-ttl-procedures-srte-paths-00

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.

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   This Internet-Draft will expire on April 19, 2019.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Problem with SR-TE Paths  . . . . . . . . . . . . . . . . . .   3
     2.1.  Short Pipe model  . . . . . . . . . . . . . . . . . . . .   3
     2.2.  Uniform Model . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Detailed Solution For TTL procedures for SR-TE paths  . . . .   5
     3.1.  P bit in DDMT TLV . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Procedures for a PHP router of the tunnel being traced  .   5
     3.3.  Procedures for a egress  router of the tunnel being
           traced  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.4.  Procedures for a ingress router of the SR-TE path . . . .   5
     3.5.  Example describing the solution . . . . . . . . . . . . .   5
   4.  Backward Compatibility  . . . . . . . . . . . . . . . . . . .   7
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

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 the LSR 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).
   c

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   Section 2 describes problems 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.

   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.

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   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)).

   4.  R2 forwards packet to R3 as ((1004,0)) (i.e.  R2 being PHP pops
   stack 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 will cause R3 to drop the packet.

   RFC 8287 suggests that when R1's LSP Echo Request has reached the
   egress of the outer tunnel, R1 should being to trace the inner tunnel
   by sending a LSP Echo Request with label stack ((1003,2),(1004,1)).
   However there is no way for R1 to do that in this scenario, because
   R1 cannot tell when the egress of the outer tunnel has been reached.

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)).

   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 traveling packet may
   not be a good practice.

   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.

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

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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.

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

   When a LSR receives an echo request it MUST validates the outermost
   FEC in the echo request.  LSR must 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 RFC8287 and RFC 8209

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

   When a LSR receives an echo request it MUST validates the outermost
   FEC in the echo request.  If LSR is egress for the outermost FEC Then
   it MUST look for the next label in the FEC stack if exists any.  If
   the LSP is the PHP router for the next FEC (next to outermost FEC in
   FEC stack if any), Then LSR MUST set 'P' bit in the downstream
   mapping TLV.  Other cases it should work as explained in RFC8287 and
   RFC 8209

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

   When an ingress LSR receives an echo response with 'P' bit set in the
   DS flags of downstream mapping TLV, 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 its
   tracing.  Other cases it should work as explained in RFC8287 and RFC
   8209

3.5.  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).

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   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 increment 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)).

   7.  R2 being PHP pops the ouetrmost label from the label stack and
   forward 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 process the echo request.

   9.  R3 validates the outermost FEC and knows that R3 is the egress
   for outermost FEC (Node-Sid R3).

   10.  Since R3 is the egress for outermost FEC so R3 should look at
   the next FEC in the FEC stack (Node-Sid-R4) and identify if R3 is the
   PHP router for next FEC in the label stack.  Since R3 is the PHP
   router for next FEC (Node-Sid R4) R3 should set 'P' bit in the in the
   DS flags in the DDMT TLV in echo response with return code as
   'Egress'.

   11.  When R1 receives 'P' in the DDMT TLV as well as return code as
   egress then R1 knows that the ouetrmost tunnel is traced.

   11.  R1 should send the next mpls LSP Echo Request with label stack
   ((1003,2),(1004,2)) with FEC Node-Sid-R4 (Since its received Egress
   for ouetrmost FEC Node-Sid-R3).

   12.  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
   there is no more FEC in the FEC stack TLV.

   16.  R4 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.

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4.  Backward Compatibility

   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] 3 P:
   Penulimate Hop router 2-0 Unassigned

7.  References

7.1.  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>.

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   [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>.

7.2.  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>.

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

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