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Versions: 00 01 02 03 04 05 06 draft-ietf-mpls-self-ping

MPLS Working Group                                              R. Torvi
Internet-Draft                                                 R. Bonica
Intended status: Informational                          Juniper Networks
Expires: April 26, 2015                                          M. Conn
                                                              D. Pacella
                                                             L. Tomotaki
                                                               M. Wygant
                                                                 Verizon
                                                        October 23, 2014


                             LSP Self-Ping
                     draft-bonica-mpls-self-ping-02

Abstract

   This memo describes LSP Self-ping.  Ingress LSR's can use LSP Self-
   ping to verify that an LSP is ready to carry traffic.

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
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   This Internet-Draft will expire on April 26, 2015.

Copyright Notice

   Copyright (c) 2014 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
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   include Simplified BSD License text as described in Section 4.e 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.  LSP Self Ping Procedures  . . . . . . . . . . . . . . . . . .   3
   3.  Rejected Approaches . . . . . . . . . . . . . . . . . . . . .   6
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   7.  Normative References  . . . . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   An ingress Label Switching Router (LSR) can use RSVP-TE [RFC3209] to
   establish an MPLS Label Switched Path [RFC3032].  The following
   paragraphs provide an overview of RSVP-TE procedures.

   The ingress LSR calculates an explicit path between itself and an
   egress LSR.  It then formats an RSVP PATH message, including an
   Explicit Route Object (ERO).  The ERO represents the explicit path
   between the ingress and egress LSRs.

   The ingress LSR forwards the PATH message in the direction of the
   egress LSR, following the path defined by the ERO.  Each transit LSR
   that receives the PATH message executes admission control procedures.
   If the transit LSR admits the LSP, it reserves bandwidth (if
   necessary) and sends the PATH message downstream, to the next node in
   the ERO.

   When the egress LSR receives the PATH message, it binds a label to
   the LSP.  The label can be implicit null, explicit null, or non-null.
   The egress LSR then installs forwarding state (if necessary), and
   constructs an RSVP RESV message.  The RESV message includes a Label
   Object containing the label that has been bound to the LSP.

   The egress LSR sends the RESV message upstream towards the ingress
   LSR.  The RESV message visits the same transit LSRs that the PATH
   message visited, but in reverse order.  Each transit LSR binds a
   label to the LSP, updates its forwarding state and updates the RESV
   message.  As a result, the RESV message contains a Label Object and
   the Label Object contains the label that has been bound to the LSP.
   Next, the transit LSR sends the RESV message upstream, along the
   explicit path.





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   The ingress LSR receives the RESV message and installs forwarding
   state.  Once the ingress LSR installs forwarding state it can forward
   traffic through the LSP.

   An implementation can optimize the procedure described above by
   allowing LSRs to send a RESV messages upstream before installing
   forwarding state.  This optimization is desirable, because it allows
   LSRs to install forwarding state in parallel, thus accelerating the
   process of LSP signaling and setup.  However, this optimization
   creates a race condition.  When the ingress LSR receives a RESV
   message, some downstream LSRs may have not yet completed the process
   of forwarding state installation.  If the ingress sends traffic over
   the LSP, the traffic will be black-holed until forwarding state has
   been installed on all downstream LSRs.

   The ingress LSP can prevent back-holing by verifying the LSPs
   readiness to carry traffic before forwarding traffic through it.
   Ingress LSRs can use LSP Self-Ping to verify that an LSP is ready to
   carry traffic.

   LSP Self-ping is an extremely lightweight mechanism, designed to
   perform well when control plane resources are scarce.  Therefore, LSP
   Self-ping consumes no control plane resources on transit or egress
   LSRs.

   This memo describes LSP Self-ping.

2.  LSP Self Ping Procedures

   In order to verify that an LSP is ready to carry traffic, the ingress
   LSR creates a short-lived LSP Self-ping session.  All session state
   is maintained locally on the ingress LSR.  Session state includes the
   following:

   o  Session-id: A 32-bit number that identifies the session

   o  verification-status: A boolean variable indicating whether LSP
      readiness has been verified.  The initial value of this variable
      is FALSE.

   o  retries: The number of times that the ingress LSR probes the LSP
      before giving up.  The initial value of this variable is
      determined by configuration.

   o  retry-timer: The number of milliseconds that the LSR waits after
      probing the LSP.  The initial value of this variable is determined
      by configuration.




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   The ingress LSR executes the following procedure until verification-
   status equals TRUE or retries is less than 1:

   o  Format a MPLS Echo [RFC4379] message

   o  Send the MPLS Echo message through the LSP under test

   o  Set a timer to expire in retry-timer milliseconds

   o  Wait until either a) a MPLS Echo message associated with the
      session returns or b) the timer expires.  If an MPLS Echo message
      associated with the session returns, set verification-status to
      TRUE.  Otherwise, decrement retries.  Optionally, increase the
      value of retry-timer according to an appropriate back off
      algorithm.

   As per [RFC4379], the MPLS Echo message is encapsulate in a User
   Datagram Protocol (UDP) [RFC0768] header.  If the protocol messages
   used to establish the LSP were delivered over IPv4 [RFC0791], the UDP
   datagram is encapsulated in an IPv4 header.  If the protocol messages
   used to establish the LSP were delivered over IPv6 [RFC2460], the UDP
   datagram is encapsulated in an IPv6 header.

   In either case, message contents are as follows:

   o  IP Source Address is configurable.  By default, it is the address
      of the egress LSR

   o  IP Destination Address is the address of the ingress LSR

   o  IP Time to Live (TTL) / Hop Count is 255

   o  IP DSCP is configurable.  By default, it is equal to CS6 (Ox48)
      [RFC4594]

   o  UDP Source Port is 3503

   o  UDP Destination Port is 3503

   o  MPLS Echo Global Flags are clear (i.e., set to 0)

   o  MPLS Echo Type is equal to "MPLS Echo Reply" (2)

   o  MPLS Echo Reply Mode is "Reply via an IPv4/IPv6 UDP packet" (2)

   o  MPLS Echo Senders Handle is equal to the Session-ID

   o  MPLS Echo Sequnce Number is equal to retries



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   The reader should note that the ingress LSR probes the LSP by sending
   an MPLS Echo message, addressed to itself, through the LSP.  The
   egress LSR forwards the MPLS Echo message back to the ingress LSR,
   exactly as it would forward any other packet.

   If the LSP under test is ready to carry traffic, the egress LSR
   receives the MPLS Echo message.  The MPLS Echo message can arrive at
   the egress LSR with or without an MPLS header, depending on whether
   the LSP under test executes penultimate hop-popping procedures.  If
   the MPLS Echo message arrives at the egress LSR with an MPLS header,
   the egress LSR removes that header.

   The egress LSR forwards the MPLS Echo message to its destination, the
   ingress LSR.  The egress LSR forwards the MPLS Echo message exactly
   as it would forward any other packet.  If the egress LSR's most
   preferred route to the ingress LSR is through an LSP, the egress LSR
   forwards the MPLS Echo message through that LSP.  However, if the
   egress LSR's most preferred route to the ingress LSR is not through
   an LSP, the egress LSR forwards the MPLS Echo message without MPLS
   encapsulation.

   If the ingress LSR receives an MPLS Echo message with Senders Handle
   equal to the Session-ID, it sets the verification-status to TRUE.
   The Sequence Number does not have to match the last Sequence Number
   sent.

   When an LSP Self-ping session terminates, it returns the value of
   verification-status to the invoking protocol.  For example, assume
   that RSVP-TE invokes LSP Self-ping as part of the LSP set-up
   procedure.  If LSP Self-ping returns TRUE, RSVP-TE makes the LSP
   under test available for forwarding.  However, if LSP Self-ping
   returns FALSE, RSVP-TE takes appropriate remedial actions.

   LSP Self-ping fails if all of the following conditions are true:

   o  The Source Address of the MPLS Echo message is equal to its
      default value (that is, the address of the egress LSR)

   o  The penultimate hop pops the MPLS label

   o  The egress LSR executes Unicast Reverse Path Forwarding (uRPF)
      procedures

   In this scenario and in similar scenarios, the egress LSR discards
   the MPLS Echo message rather than forwarding it.  In such scenarios,
   the calling application can set the source address to a more
   appropriate value.




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3.  Rejected Approaches

   In a rejected approach, the ingress LSR uses LSP-Ping, exactly as
   described in [RFC4379] to verify LSP readiness to carry traffic.
   This approach was rejected for the following reasons.

   While an ingress LSR can control its control plane overhead due to
   LSP Ping, an egress LSR has no such control.  This is because each
   ingress LSR can, on its own, control the rate of the LSP Ping
   originated by the LSR, while an egress LSR must respond to all the
   LSP Pings originated by various ingresses.  Furthermore, when an MPLS
   Echo Request reaches an egress LSR it is sent to the control plane of
   the egress LSR, which makes egress LSR processing overhead of LSP
   Ping well above the overhead of its data plane (MPLS/IP forwarding).
   These factors make LSP Ping problematic as a tool for detecting LSP
   readiness to carry traffic when dealing with a large number of LSPs.

   By contrast, LSP Self-ping does not consume any control plane
   resources at the egress LSR, and relies solely on the data plane of
   the egress LSR, making it more suitable as a tool for checking LSP
   readiness when dealing with a large number of LSPs.

   In another rejected approach, the ingress LSR does not verify LSP
   readiness.  Alternatively, it sets a timer when it receives an RSVP
   RESV message and does not forward traffic through the LSP until the
   timer expires.  This approach was rejected because it is impossible
   to determine the optimal setting for this timer.  If the timer value
   is set too low, it does not prevent black-holing.  If the timer value
   is set too high, it slows down the process of LSP signalling and
   setup.

   Moreover, the above-mentioned timer is configured on a per-router
   basis.  However, its optimum value is determined by a network-wide
   behavior.  Therefore, changes in the network could require changes to
   the value of the timer, making the optimal setting of this timer a
   moving target.

4.  IANA Considerations

   This document makes no request of IANA.

   Note to RFC Editor: this section may be removed on publication as an
   RFC.








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5.  Security Considerations

   MPLS Echo messages are easily forged.  Therefore, an attacker can
   send the ingress LSR a forged MPLS Echo message, causing the ingress
   LSR to terminate the LSP Self-ping session prematurely.

6.  Acknowledgements

   Thanks to Yakov Rekhter, Ravi Singh, Eric Rosen, Eric Osborne and
   Nobo Akiya for their contributions to this document.

7.  Normative References

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              August 1980.

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791, September
              1981.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

   [RFC3032]  Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
              Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
              Encoding", RFC 3032, January 2001.

   [RFC3209]  Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
              and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
              Tunnels", RFC 3209, December 2001.

   [RFC4379]  Kompella, K. and G. Swallow, "Detecting Multi-Protocol
              Label Switched (MPLS) Data Plane Failures", RFC 4379,
              February 2006.

   [RFC4594]  Babiarz, J., Chan, K., and F. Baker, "Configuration
              Guidelines for DiffServ Service Classes", RFC 4594, August
              2006.

Authors' Addresses

   Ravi Torvi
   Juniper Networks

   Email: rtorvi@juniper.net







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   Ron Bonica
   Juniper Networks

   Email: rbonica@juniper.net


   Michael Conn
   Verizon

   Email: michael.e.conn@verizon.com


   Dante Pacella
   Verizon

   Email: dante.j.pacella@verizon.com


   Luis Tomotaki
   Verizon

   Email: luis.tomotaki@verizon.com


   Mark Wygant
   Verizon

   Email: mark.wygant@verizon.com























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