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Versions: (draft-osborne-mpls-psc-updates) 00 01 02 03 04 05 06 RFC 7324

Network Working Group                                         E. Osborne
Internet-Draft                                             Cisco Systems
Intended status: Standards Track                        October 21, 2013
Expires: April 24, 2014


                             Updates to PSC
                     draft-ietf-mpls-psc-updates-00

Abstract

   This document contains four updates to the Protection State
   Coordination (PSC) logic defined in RFC6378, "MPLS Transport Profile
   (MPLS-TP) Linear Protection" . Two of the updates correct existing
   behavior.  The third clears up a behavior which was not explained in
   the RFC, and the fourth adds rules around handling capabilities
   mismatches.

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 http://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."

   This Internet-Draft will expire on April 24, 2014.

Copyright Notice

   Copyright (c) 2013 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



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   (http://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
   to this document.  Code Components extracted from this document must
   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Incorrect local status after failure  . . . . . . . . . . . .   2
   3.  Reversion deadlock due to a race condition  . . . . . . . . .   3
   4.  Clarifying PSC's behavior in the face of multiple inputs  . .   4
   5.  Handling a capabilities mismatch  . . . . . . . . . . . . . .   6
     5.1.  PT mismatch . . . . . . . . . . . . . . . . . . . . . . .   6
     5.2.  R mismatch  . . . . . . . . . . . . . . . . . . . . . . .   6
     5.3.  Unsupported modes . . . . . . . . . . . . . . . . . . . .   7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   This document contains four updates to PSC [RFC6378].  Three of them
   fix issues #2, #7 and #8 as identified in the ITU's liaison statement
   "Recommendation ITU-T G.8131/Y.1382 revision - Linear protection
   switching for MPLS-TP networks" [LIAISON].  The fourth clears up a
   behavior which was not well explained in RFC6378.  These updates are
   not changes to the protocol's packet format or to PSC's design, but
   are corrections and clarifications to specific aspects of the
   protocol's procedures.

2.  Incorrect local status after failure

   Issue #2 in the liaison identifies a case where a strict reading of
   RFC6378 leaves a node reporting an inaccurate status:

   . A node can end up sending incorrect status - NR(0,1) - despite the
   failure of the protection LSP (P-LSP).  This is clearly not correct,
   as a node should not be sending NR if it has a local failure.  To
   address this issue, the fourth bullet in section 4.3.3.3 is replaced
   with the following three bullets:




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   o  If the current state is due to a local or remote Manual Switch, a
      local Signal Fail indication on the protection path SHALL cause
      the LER to enter local Unavailable state and begin transmission of
      an SF(0,0) message.

   o  If the LER is in local Protecting Administrative state due to a
      local Forced Switch, a local Signal Fail indication on the
      protection path SHALL be ignored.

   o  If the LER is in remote Protecting Administrative state due to a
      remote Forced Switch, a local Signal Fail indication on the
      protection path SHALL cause the LER to remain in remote Protecting
      administrative state and transmit an SF(0,1) message.

3.  Reversion deadlock due to a race condition

   Issue #8 in the liaison identifies a deadlock case where each node
   can end up sending NR(0,1) when it should instead be in the process
   of recovering from the failure (i.e. entering into WTR or DNR, as
   appropriate for the protection domain).  The root of the issue is
   that a pair of nodes can simultaneously enter WTR state, receive an
   out of date SF-W indication and transition into a remotely triggered
   WTR, and remain in remotely triggered WTR waiting for the other end
   to trigger a change in status.

   In the case identified in issue #8, each node can end up sending
   NR(0,1), which is an indication that the transmitting node has no
   local failure, but is instead reacting to the remote SF-W. If a node
   which receives NR(0,1) is in fact not indicating a local error, the
   receive node can take the received NR(0,1) as an indication that
   there is no error in the protection domain, and recovery procedures
   (WTR or DNR) should begin.

   This is addressed by adding the following text as the penultimate
   bullet in section 4.3.3.4:

   o  If a node is in Protecting Failure state due to a remote SF-W and
      receives NR(0,1), this SHALL cause the node to begin recovery
      procedures.  If the LER is configured for revertive behavior, it
      enters into Wait-to-Restore state, starts the WTR timer, and
      begins transmitting WTR(0,1).  If the LER is configured for non-
      revertive behavior, it enters into Do-Not-Revert state and begins
      transmitting a DNR(0,1) message.

   Additionally, the final bullet in section 4.3.3.3 is changed from

   o  A remote NR(0,0) message SHALL be ignored if in local Protecting
      administrative state.



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   to

   o  A remote No Request message SHALL be ignored if in local
      Protecting administrative state.

   This indicates that a remote NR triggers the same behavior regardless
   of the value of FPath and Path.  This change does not directly
   address issue #8, but fixes a similar issue - if a node receives NR
   while in Remote administrative state, the value of FPath and Path
   have no bearing on the node's reaction to this NR.

4.  Clarifying PSC's behavior in the face of multiple inputs

   RFC6378 describes the PSC state machine.  Figure 1 in section 3 shows
   two inputs into the PSC Control logic - Local Request logic and
   Remote PSC Request.  When there is only one input into the PSC
   Control logic - a local request or a remote request but not both -
   the PSC Control logic decides what that input signifies and then
   takes one or more actions, as necessary.  This is what the PSC State
   Machine in section 4.3 describes.

   RFC6378 does not sufficiently describe the behavior in the face of
   multiple inputs into the PSC Control Logic (one Local Request and one
   Remote Request).  This section clarifies the expected behavior.

   There are two cases to think about when considering dual inputs into
   the PSC Control logic.  The first is when the same request is
   presented from both local and remote sources.  One example of this
   case is a Forced Switch (FS) configured on both ends of an LSP.  This
   will result in the PSC Control logic receiving both a local FS and
   remove FS.  For convenience, this scenario is written as [L(FS),
   R(FS))]

   The second case, which is handled in exactly the same way as the
   first, is when the two inputs into the PSC Control logic describe
   different events.  There are a number of variations on this case.
   One example is when there is a Lockout of Protection from the Local
   request logic and a Forced Switch from the Remote PSC Request.  This
   is shortened to [L(LO), R(FS)].

   In both cases the question is not how the PSC Control logic decides
   which of these is the one it acts upon.  Section 4.3.2 of RFC6378
   lists the priority order, and prioritizes the local input over the
   remote input in case both inputs are of the same priority.  So in the
   first example it is the local SF that drives the PSC Control logic,
   and in the second example it is the local Lockout which drives the
   PSC Control logic.




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   The point that this section clears up is around what happens when the
   highest priority input goes away.  Consider the first case.
   Initially, the PSC Control logic has [L(FS), R(FS)] and L(FS) is
   driving PSC's behavior.  When L(FS) is removed but R(FS) remains,
   what does PSC do?  A strict reading of the FSM would suggest that PSC
   transition from PA:F:L into N, and at some future time (perhaps after
   the remote request refreshes) PSC would transition from N to PA:F:R.
   This is an unreasonable behavior, as there is no sensible
   justification for a node behaving as if things were normal (i.e. N
   state) when it is clear that they are not.

   The second case is similar.  If a node starts with [L(LO), R(FS)] and
   the local lockout is removed, a strict reading of the state machine
   would suggest that the node transition from UA:LO:L to N, and then at
   some future time presumably notice the R(FS) and transition from N to
   PA:F:R. As with the first case, this is clearly not a useful
   behavior.

   In both cases the request which was driving PSC's behavior was
   removed.  What should happen is that the PSC Control logic should,
   upon removal of an input, immediately reevaluate all other inputs to
   decide on the next course of action.  This requires an implementation
   to store the most recent local and remote inputs regardless of their
   eventual use as triggers for the PSC Control Logic.

   There is a third case.  Consider a node with [L(FS), R(LO)].  At some
   point in time the remote node replaces its Lockout request with a
   Signal Fail on Working, so that the inputs into the PSC Control logic
   on the receiving node go to [L(FS), R(SF-W)].  Similar to the first
   two cases, the node should immediately reevaluate both its local and
   remote inputs to determine the highest priority among them, and act
   on that input accordingly.  That is in fact what happens, as defined
   in Section 4.3.3:

   "When a LER is in a remote state, i.e., state transition in reaction
   to a PSC message received from the far-end LER, and receives a new
   PSC message from the far-end LER that indicates a contradictory
   state, e.g., in remote Unavailable state receiving a remote FS(1,1)
   message, then the PSC Control logic SHALL reevaluate all inputs (both
   the local input and the remote message) as if the LER is in the
   Normal state."

   This section extends that paragraph to handle the first two cases.
   The essence of the quoted paragraph is that when faced with multiple
   inputs, PSC must reevaluate any changes as if it was in Normal state.
   So the quoted paragraph is replaced with the following text:





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   "The PSC Control logic may simultaneously have Local and Remote
   requests, and the highest priority of these requests ultimately
   drives the behavior of the PSC Control logic.  When this highest
   priority request is removed or is replaced with another input, then
   the PSC Control logic SHALL immediately reevaluate all inputs (both
   the local input and the remote message), transitioning into a new
   state only upon reevaluation of all inputs".

5.  Handling a capabilities mismatch

   PSC has no explicit facility to negotiate any properties of the
   protection domain.  It does, however, have the ability to signal two
   properties of that domain, via the Protection Type (PT) and Revertive
   (R) bits.  RFC6378 specifies that if these bits do not match an
   operator "SHALL [be notified]" (PT, section 4.2.3) or "SHOULD be
   notified" (R, section 4.2.4).  However, there is no text which
   specifies the behavior of the end nodes of a protection domain in
   case of a mismatch.  This section provides that text, as requested by
   issue #7 in the liaison.

5.1.  PT mismatch

   The behavior of the protection domain depends on the exact PT
   mismatch.  Section 4.2.3 of RFC6378 specifies three protection types
   - bidirectional switching using a permanent bridge, bidirectional
   switching using a selector bridge, and unidirectional switching using
   a permanent bridge.  They are abbreviated here as BP, BS and UP.

   There are three possible mismatches: [BP, UP], [BP, BS], and [UP,
   BS].  The priority is:

   UP > BS > BP

   In other words:

   o  If the PT mismatch is {BP, UP}, the node transmitting BP MUST
      switch to UP mode if it is supported.

   o  If the PT mismatch is {BP, BS}, the node transmitting BP MUST
      switch to BS mode if it is supported.

   o  If the PT mismatch is {UP, BS}, the node transmitting BS MUST
      switch to UP mode if it is supported.

5.2.  R mismatch






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   The R bit indicates whether the protection domain is in Revertive or
   Non-Revertive behavior.  If the R bits do not match, the node
   indicating Non-Revertive MUST switch to Revertive if it is supported.

5.3.  Unsupported modes

   An implementation may not support all three PT modes and/or both R
   modes, and thus a pair of nodes may be unable to converge on a common
   mode.  This creates a permanent mismatch, resolvable only by operator
   intervention.  An implementation SHOULD alert the operator to an
   irreconcilable mismatch.

   It is desirable to allow the protection domain to function in a non-
   failure mode even if there is a mismatch, as the mismatches of PT or
   R have to do with how nodes recover from a failure.  An
   implementation SHOULD allow traffic to be sent on the Working LSP as
   long as there is no failure (e.g. NR state) regardless of any PT or R
   mismatch.

   If there is a trigger which would cause the protection LSP to be
   used, such as SF or MS, a node MUST NOT use the protection LSP to
   carry traffic.

6.  Security Considerations

   These changes and clarifications raise no new security concerns.

7.  IANA Considerations

   There are no requests for IANA actions in this document..

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

8.  Acknowledgements

   The author of this document thanks Taesik Cheung, Alessandro
   D'Alessandro, Annamaria Fulignoli, Sagar Soni, George Swallow and
   Yaacov Weingarten for their contributions and review.

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.





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   [RFC6378]  Weingarten, Y., Bryant, S., Osborne, E., Sprecher, N., and
              A. Fulignoli, "MPLS Transport Profile (MPLS-TP) Linear
              Protection", RFC 6378, October 2011.

   [RFC6428]  Allan, D., Swallow Ed. , G., and J. Drake Ed. , "Proactive
              Connectivity Verification, Continuity Check, and Remote
              Defect Indication for the MPLS Transport Profile", RFC
              6428, November 2011.

9.2.  Informative References

   [LIAISON]  ITU-T SG15, "Liaison Statement: Recommendation ITU-T
              G.8131/Y.1382 revision - Linear protection switching for
              MPLS-TP networks", , <https://datatracker.ietf.org/liaison
              /1205/>.

Author's Address

   Eric Osborne
   Cisco Systems

   Email: eosborne@cisco.com





























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