Internet DRAFT - draft-osborne-mpls-psc-updates
draft-osborne-mpls-psc-updates
Network Working Group E. Osborne
Internet-Draft Cisco Systems
Intended status: Standards Track October 04, 2013
Expires: April 07, 2014
Updates to PSC
draft-osborne-mpls-psc-updates-03
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
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This Internet-Draft will expire on April 07, 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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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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