LDP behaviour on link-shut scenarios
draft-aa-ldp-link-shut-01

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Network                                                         M. Anush
Internet-Draft                                              T. Anupkumar
Updates: 5036 (if approved)                                     Ericsson
Intended status: Standards Track                         August 21, 2019
Expires: February 22, 2020

                  LDP behaviour on link-shut scenarios
                       draft-aa-ldp-link-shut-01

Abstract

   This document is intended as clarification of LDP behaviour in link-
   down scenarios.  Base LDP RFC5036 lacks sufficient clarity on what an
   LDP enabled node should be doing when a link down event is received,
   and the only LDP adjacency for an LDP peer is over this link.
   Different vendors have handled this scenario differently, with some
   immediately resetting tcp session with neighbor and some waiting for
   igp recovergence instead of reacting directly to link events.  With
   this document we intend to clarify the expected behaviour explicitly
   so that any interop issues can be avoided.

Status of This Memo

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   This Internet-Draft will expire on February 22, 2020.

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   carefully, as they describe your rights and restrictions with respect
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Problem Description . . . . . . . . . . . . . . . . . . . . .   2
   4.  Solutions . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   6.  IANA Considerationss  . . . . . . . . . . . . . . . . . . . .   4
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   4
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .   4
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   4

1.  Introduction

   [RFC5036] details LDP specification and procedures to be followed by
   LDP implementations.  However, for some scenarios like link down, the
   rfc isn't particularly clear as to what an implementation is supposed
   to do.  This could lead to interop issues when routers from different
   vendors are part of the network.  More details are given in the
   problem description section below.  A possible solution is also
   suggested in the subsequent section.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   o  LDP: Label distribution protocol.

   o  GR: Graceful-restart.

3.  Problem Description

   Consider the following topology:

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                   E
                   |
                   |
                   A --- B
                   |     |
                   |     |
                   C --- D

                    Figure 1: Example topology for LDP

   All the nodes here are LDP enabled and also support GR.  We have an
   lsp from C-E via C-A-E path(this is igp bestpath).  IGP has also
   computed LFA backup for this primary-lsp via C-D-B-A-E path and we
   have LDP lfa backup as well (taking this path).

   Now, lets bring down A-C link.  Node A has detected link-shut event
   and since this link is the only adjacency to LDP-neighbor C, it
   resets the LDP session and sends shutdown to neighbour C.

   At C, the link-down event is detected bit late and subsequently the
   IGP update is also delayed.  Meanwhile, C has received shutdown from
   peer A, and it results in C flushing all labels received from A.
   Since the primary-label for C-E lsp is no longer available (from A),
   the lsp itself is deleted by LDP, as LDP can't be congruent with IGP.
   This LDP-lsp flap can in turn impact l3vpn/l2vpn traffic which are
   dependent on this LSP.

   We can definitely reduce traffic-loss by running BFD and switching
   traffic to lfa backup in forwarding, but the intention above is to
   highlight that IGP updates and subsequently LDP updates would be
   asynchronous at nodes A and C, which may be more prominent if there
   are routers with different capabilities (and maybe from different
   vendors) in the network.  So even if traffic has moved to lfa-backup
   lsp in forwarding, the primary-lsp itself could be deleted by the
   shutdown message (which is a fatal error).

4.  Solutions

   When a node has LDP adjacency to its neighbour (With GR [RFC3478]
   enabled on both nodes) over a 'single' directly connected link and
   that link goes down, the node MAY reset the tcp session with
   neighbor.  However, it MUST NOT send shutdown message, which flushes
   advertised labels at neighbor immediately.

   The neighbor itself could have different backup mechanisms (ldp-lfa,
   rsvp-bypass etc) to ensure minimal traffic loss in forwarding for
   lsps having this node as active(primary)-path.  Transmitting shutdown

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   message immediately could result in neighbor prematurely deleting
   LSPs instead of letting IGP recoverge.

   Another approach could be to avoid reacting immediately to link down
   events.  Instead, let hello timeout bringdown the session and update
   LSP-paths as soon as IGP reconverges.

   Both approaches can help to avoid traffic loss by accounting for
   asynchronous ordering of events in LDP-peering routers.

5.  Security Considerations

   The security considerations described in RFC5036 apply to this
   document.

6.  IANA Considerationss

7.  Acknowledgments

   .

8.  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>.

   [RFC3478]  Leelanivas, M., Rekhter, Y., and R. Aggarwal, "Graceful
              Restart Mechanism for Label Distribution Protocol",
              RFC 3478, DOI 10.17487/RFC3478, February 2003,
              <https://www.rfc-editor.org/info/rfc3478>.

   [RFC5036]  Andersson, L., Ed., Minei, I., Ed., and B. Thomas, Ed.,
              "LDP Specification", RFC 5036, DOI 10.17487/RFC5036,
              October 2007, <https://www.rfc-editor.org/info/rfc5036>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

Authors' Addresses

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   Anush Mohan
   Ericsson
   Bangalore
   India

   Email: anush.mohan@ericsson.com

   Anupkumar T
   Ericsson
   Bangalore
   India

   Email: anupkumar.t@ericsson.com

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