Routing Area Working Group S. Litkowski
Internet-Draft B. Decraene
Intended status: Standards Track Orange
Expires: December 23, 2017 C. Filsfils
Cisco Systems
P. Francois
Individual
June 21, 2017
Micro-loop prevention by introducing a local convergence delay
draft-ietf-rtgwg-uloop-delay-05
Abstract
This document describes a mechanism for link-state routing protocols
to prevent local transient forwarding loops in case of link failure.
This mechanism proposes a two-steps convergence by introducing a
delay between the convergence of the node adjacent to the topology
change and the network wide convergence.
As this mechanism delays the IGP convergence it may only be used for
planned maintenance or when fast reroute protects the traffic between
the link failure time and the IGP convergence.
The proposed mechanism will be limited to the link down event in
order to keep simplicity.
Simulations using real network topologies have been performed and
show that local loops are a significant portion (>50%) of the total
forwarding loops.
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 [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/.
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This Internet-Draft will expire on December 23, 2017.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Transient forwarding loops side effects . . . . . . . . . . . 3
2.1. Fast reroute inefficiency . . . . . . . . . . . . . . . . 4
2.2. Network congestion . . . . . . . . . . . . . . . . . . . 6
3. Overview of the solution . . . . . . . . . . . . . . . . . . 7
4. Specification . . . . . . . . . . . . . . . . . . . . . . . . 7
4.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . 7
4.2. Current IGP reactions . . . . . . . . . . . . . . . . . . 8
4.3. Local events . . . . . . . . . . . . . . . . . . . . . . 8
4.4. Local delay for link down . . . . . . . . . . . . . . . . 9
5. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 9
5.1. Applicable case: local loops . . . . . . . . . . . . . . 9
5.2. Non applicable case: remote loops . . . . . . . . . . . . 10
6. Simulations . . . . . . . . . . . . . . . . . . . . . . . . . 10
7. Deployment considerations . . . . . . . . . . . . . . . . . . 11
8. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 12
8.1. Local link down . . . . . . . . . . . . . . . . . . . . . 12
8.2. Local and remote event . . . . . . . . . . . . . . . . . 15
8.3. Aborting local delay . . . . . . . . . . . . . . . . . . 17
9. Comparison with other solutions . . . . . . . . . . . . . . . 19
9.1. PLSN . . . . . . . . . . . . . . . . . . . . . . . . . . 19
9.2. OFIB . . . . . . . . . . . . . . . . . . . . . . . . . . 20
10. Existing implementations . . . . . . . . . . . . . . . . . . 20
11. Security Considerations . . . . . . . . . . . . . . . . . . . 21
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12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 21
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
14.1. Normative References . . . . . . . . . . . . . . . . . . 21
14.2. Informative References . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
1. Introduction
Micro-forwarding loops and some potential solutions are well
described in [RFC5715]. This document describes a simple targeted
mechanism that solves micro-loops that are local to the failure;
based on network analysis, these are a significant portion of the
micro-forwarding loops. A simple and easily deployable solution for
these local micro-loops is critical because these local loops cause
some traffic loss after a fast-reroute alternate has been used (see
Section 2.1).
Consider the case in Figure 1 where S does not have an LFA to protect
its traffic to D. That means that all non-D neighbors of S on the
topology will send to S any traffic destined to D if a neighbor did
not, then that neighbor would be loop-free. Regardless of the
advanced fast-reroute (FRR) technique used, when S converges to the
new topology, it will send its traffic to a neighbor that was not
loop-free and thus cause a local micro-loop. The deployment of
advanced fast-reroute techniques motivates this simple router-local
mechanism to solve this targeted problem. This solution can be work
with the various techniques described in [RFC5715].
1
D ------ C
| |
1 | | 5
| |
S ------ B
1
Figure 1
When S-D fails, a transient forwarding loop may appear between S and
B if S updates its forwarding entry to D before B.
2. Transient forwarding loops side effects
Even if they are very limited in duration, transient forwarding loops
may cause high damages for a network.
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2.1. Fast reroute inefficiency
D
1 |
| 1
A ------ B
| | ^
10 | | 5 | T
| | |
E--------C
| 1
1 |
S
Figure 2 - RSVP-TE FRR case
In the Figure 2, an RSVP-TE tunnel T, provisioned on C and
terminating on B, is used to protect against C-B link failure (IGP
shortcut is activated on C). The primary path of T is C->B and FRR
is activated on T providing an FRR bypass or detour using path
C->E->A->B. On the router C, the nexthop to D is the tunnel T thanks
to the IGP shortcut. When C-B link fails:
1. C detects the failure, and updates the tunnel path using
preprogrammed FRR path, the traffic path from S to D becomes:
S->E->C->E->A->B->A->D.
2. In parallel, on router C, both the IGP convergence and the TE
tunnel convergence (tunnel path recomputation) are occurring:
* The Tunnel T path is recomputed and now uses C->E->A->B.
* The IGP path to D is recomputed and now uses C->E->A->D.
3. On C, the tail-end of the TE tunnel (router B) is no more on the
shortest-path tree (SPT) to D, so C does not encapsulate anymore
the traffic to D using the tunnel T and updates its forwarding
entry to D using the nexthop E.
If C updates its forwarding entry to D before router E, there would
be a transient forwarding loop between C and E until E has converged.
+-----------+------------+------------------+-----------------------+
| Network | Time | Router C events | Router E events |
| condition | | | |
+-----------+------------+------------------+-----------------------+
| S->D | | | |
| Traffic | | | |
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| OK | | | |
| | | | |
| S->D | t0 | Link B-C fails | Link B-C fails |
| Traffic | | | |
| lost | | | |
| | | | |
| | t0+20msec | C detects the | |
| | | failure | |
| | | | |
| S->D | t0+40msec | C activates FRR | |
| Traffic | | | |
| OK | | | |
| | | | |
| | t0+50msec | C updates its | |
| | | local LSP/LSA | |
| | | | |
| | t0+60msec | C schedules SPF | |
| | | (100ms) | |
| | | | |
| | t0+70msec | C floods its | |
| | | local updated | |
| | | LSP/LSA | |
| | | | |
| | t0+87msec | | E receives LSP/LSA |
| | | | from C and schedules |
| | | | SPF (100ms) |
| | | | |
| | t0+117msec | | E floods LSP/LSA from |
| | | | C |
| | | | |
| | t0+160msec | C computes SPF | |
| | | | |
| | t0+165msec | C starts | |
| | | updating its | |
| | | RIB/FIB | |
| | | | |
| | t0+193msec | | E computes SPF |
| | | | |
| | t0+199msec | | E starts updating its |
| | | | RIB/FIB |
| | | | |
| S->D | t0+255msec | C updates its | |
| Traffic | | RIB/FIB for D | |
| lost | | | |
| | | | |
| | t0+340msec | C convergence | |
| | | ends | |
| | | | |
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| S->D | t0+443msec | | E updates its RIB/FIB |
| Traffic | | | for D |
| OK | | | |
| | | | |
| | t0+470msec | | E convergence ends |
+-----------+------------+------------------+-----------------------+
Route computation event time scale
The issue described here is completely independent of the fast-
reroute mechanism involved (TE FRR, LFA/rLFA, MRT ...). The
protection enabled by fast-reroute is working perfectly, but ensures
a protection, by definition, only until the PLR has converged. When
implementing FRR, a service provider wants to guarantee a very
limited loss of connectivity time. The previous example shows that
the benefit of FRR may be completely lost due to a transient
forwarding loop appearing when PLR has converged. Delaying FIB
updates after the IGP convergence may allow to keep the fast-reroute
path until the neighbors have converged and preserves the customer
traffic.
2.2. Network congestion
1
D ------ C
| |
1 | | 5
| |
A -- S ------ B
/ | 1
F E
In the figure above, as presented in Section 1, when the link S-D
fails, a transient forwarding loop may appear between S and B for
destination D. The traffic on the S-B link will constantly increase
due to the looping traffic to D. Depending on the TTL of the
packets, the traffic rate destinated to D and the bandwidth of the
link, the S-B link may be congested in few hundreds of milliseconds
and will stay overloaded until the loop is solved.
The congestion introduced by transient forwarding loops is
problematic as it is impacting traffic that is not directly concerned
by the failing network component. In our example, the congestion of
the S-B link will impact some customer traffic that is not directly
concerned by the failure: e.g. A to B, F to B, E to B. Some class
of services may be implemented to mitigate the congestion, but some
traffic not directly concerned by the failure would still be dropped
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as a router is not able to identify the looping traffic from the
normally forwarded traffic.
3. Overview of the solution
This document defines a two-step convergence initiated by the router
detecting the failure and advertising the topological changes in the
IGP. This introduces a delay between the convergence of the local
router and the network wide convergence.
The proposed solution is kept limited to local link down events for
simplicity reason.
This ordered convergence, is similar to the ordered FIB proposed
defined in [RFC6976], but limited to only a "one hop" distance. As a
consequence, it is simpler and becomes a local only feature not
requiring interoperability; at the cost of only covering the
transient forwarding loops involving this local router. The proposed
mechanism also reuses some concept described in
[I-D.ietf-rtgwg-microloop-analysis] with some limitations.
4. Specification
4.1. Definitions
This document will refer to the following existing IGP timers:
o LSP_GEN_TIMER: The delay used to batch multiple local events in
one single local LSP/LSA update. It is often associated with a
damping mechanism to slow down reactions by incrementing the timer
when multiple consecutive events are detected.
o SPF_DELAY: The delay between the first IGP event triggering a new
routing table computation and the start of that routing table
computation. It is often associated with a damping mechanism to
slow down reactions by incrementing the timer when the IGP becomes
unstable. As an example, [I-D.ietf-rtgwg-backoff-algo] defines a
standard SPF delay algorithm.
This document introduces the following new timer:
o ULOOP_DELAY_DOWN_TIMER: used to slow down the local node
convergence in case of link down events.
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4.2. Current IGP reactions
Upon a change of the status of an adjacency/link, the existing
behavior of the router advertising the event is the following:
1. The Up/Down event is notified to the IGP.
2. The IGP processes the notification and postpones the reaction in
LSP_GEN_TIMER msec.
3. Upon LSP_GEN_TIMER expiration, the IGP updates its LSP/LSA and
floods it.
4. The SPF computation is scheduled in SPF_DELAY msec.
5. Upon SPF_DELAY expiration, the SPF is computed, then the RIB and
FIB are updated.
4.3. Local events
The mechanism described in this document assumes that there has been
a single link failure as seen by the IGP area/level. If this
assumption is violated (e.g. multiple links or nodes failed), then
standard IP convergence MUST be applied (as described in
Section 4.2).
To determine if the mechanism can be applicable or not, an
implementation SHOULD implement a logic to correlate the protocol
messages (LSP/LSA) received during the SPF scheduling period in order
to determine the topology changes that occured. This is necessary as
multiple protocol messages may describe the same topology change and
a single protocol message may describe multiple topology changes. As
a consequence, determining a particular topology change MUST be
independent of the order of reception of those protocol messages.
How the logic works is let to implementation details.
Using this logic, if an implementation determines that the associated
topology change is a single local link failure, then the router MAY
use the mechanism described in this document, otherwise the standard
IP convergence MUST be used.
Example:
+--- E ----+--------+
| | |
A ---- B -------- C ------ D
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Let router B be the computing router when the link B-C fails. B
updates its local LSP/LSA describing the link B->C as down, C does
the same, and both start flooding their updated LSP/LSAs. During the
SPF_DELAY period, B and C learn all the LSPs/LSAs to consider. B
sees that C is flooding as down a link where B is the other end and
that B and C are describing the same single event. Since B receives
no other changes, B can determine that this is a local link failure
and may decide to activate the mechanism described in this document.
4.4. Local delay for link down
Upon an adjacency/link down event, this document introduces a change
in step 5 (Section 4.2) in order to delay the local convergence
compared to the network wide convergence: the node SHOULD delay the
forwarding entry updates by ULOOP_DELAY_DOWN_TIMER. Such delay
SHOULD only be introduced if all the LSDB modifications processed are
only reporting a single local link down event (Section 4.3). If a
subsequent LSP/LSA is received/updated and a new SPF computation is
triggered before the expiration of ULOOP_DELAY_DOWN_TIMER, then the
same evaluation SHOULD be performed.
As a result of this addition, routers local to the failure will
converge slower than remote routers. Hence it SHOULD only be done
for a non-urgent convergence, such as for administrative de-
activation (maintenance) or when the traffic is protected by fast-
reroute.
5. Applicability
As previously stated, the mechanism only avoids the forwarding loops
on the links between the node local to the failure and its neighbor.
Forwarding loops may still occur on other links.
5.1. Applicable case: local loops
A ------ B ----- E
| / |
| / |
G---D------------C F All the links have a metric of 1
Figure 2
Let us consider the traffic from G to F. The primary path is
G->D->C->E->F. When link C-E fails, if C updates its forwarding
entry for F before D, a transient loop occurs. This is sub-optimal
as C has FRR enabled and it breaks the FRR forwarding while all
upstream routers are still forwarding the traffic to itself.
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By implementing the mechanism defined in this document on C, when the
C-E link fails, C delays the update of its forwarding entry to F, in
order to let some time for D to converge. FRR keeps protecting the
traffic during this period. When the timer expires on C, its
forwarding entry to F is updated. There is no transient forwarding
loop on the link C-D.
5.2. Non applicable case: remote loops
A ------ B ----- E --- H
| |
| |
G---D--------C ------F --- J ---- K
All the links have a metric of 1 except BE=15
Figure 3
Let us consider the traffic from G to K. The primary path is
G->D->C->F->J->K. When the C-F link fails, if C updates its
forwarding entry to K before D, a transient loop occurs between C and
D.
By implementing the mechanism defined in this document on C, when the
link C-F fails, C delays the update of its forwarding entry to K,
letting time for D to converge. When the timer expires on C, its
forwarding entry to F is updated. There is no transient forwarding
loop between C and D. However, a transient forwarding loop may still
occur between D and A. In this scenario, this mechanism is not
enough to address all the possible forwarding loops. However, it
does not create additional traffic loss. Besides, in some cases
-such as when the nodes update their FIB in the following order C, A,
D, for example because the router A is quicker than D to converge-
the mechanism may still avoid the forwarding loop that was occurring.
6. Simulations
Simulations have been run on multiple service provider topologies.
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+----------+------+
| Topology | Gain |
+----------+------+
| T1 | 71% |
| T2 | 81% |
| T3 | 62% |
| T4 | 50% |
| T5 | 70% |
| T6 | 70% |
| T7 | 59% |
| T8 | 77% |
+----------+------+
Table 1: Number of Repair/Dst that may loop
We evaluated the efficiency of the mechanism on eight different
service provider topologies (different network size, design). The
benefit is displayed in the table above. The benefit is evaluated as
follows:
o We consider a tuple (link A-B, destination D, PLR S, backup
nexthop N) as a loop if upon link A-B failure, the flow from a
router S upstream from A (A could be considered as PLR also) to D
may loop due to convergence time difference between S and one of
his neighbor N.
o We evaluate the number of potential loop tuples in normal
conditions.
o We evaluate the number of potential loop tuples using the same
topological input but taking into account that S converges after
N.
o The gain is how much loops (remote and local) we succeed to
suppress.
On topology 1, 71% of the transient forwarding loops created by the
failure of any link are prevented by implementing the local delay.
The analysis shows that all local loops are obviously solved and only
remote loops are remaining.
7. Deployment considerations
Transient forwarding loops have the following drawbacks:
o They limit FRR efficiency: even if FRR is activated in 50msec, as
soon as PLR has converged, the traffic may be affected by a
transient loop.
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o They may impact traffic not directly concerned by the failure (due
to link congestion).
This local delay proposal is a transient forwarding loop avoidance
mechanism (like OFIB). Even if it only addresses local transient
loops, the efficiency versus complexity comparison of the mechanism
makes it a good solution. It is also incrementally deployable with
incremental benefits, which makes it an attractive option for both
vendors to implement and Service Providers to deploy. Delaying the
convergence time is not an issue if we consider that the traffic is
protected during the convergence.
8. Examples
We will consider the following figure for the associated examples :
D
1 | F----X
| 1 |
A ------ B
| | ^
10 | | 5 | T
| | |
E--------C
| 1
1 |
S
The network above is considered to have a convergence time about 1
second, so ULOOP_DELAY_DOWN_TIMER will be adjusted to this value. We
also consider that FRR is running on each node.
8.1. Local link down
The table below describes the events and associating timing that
happens on router C and E when link B-C goes down. As C detects a
single local event corresponding to a link down (its LSP + LSP from B
received), it decides to apply the local delay down behavior and no
microloop is formed.
+-----------+-------------+------------------+----------------------+
| Network | Time | Router C events | Router E events |
| condition | | | |
+-----------+-------------+------------------+----------------------+
| S->D | | | |
| Traffic | | | |
| OK | | | |
| | | | |
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| S->D | t0 | Link B-C fails | Link B-C fails |
| Traffic | | | |
| lost | | | |
| | | | |
| | t0+20msec | C detects the | |
| | | failure | |
| | | | |
| S->D | t0+40msec | C activates FRR | |
| Traffic | | | |
| OK | | | |
| | | | |
| | t0+50msec | C updates its | |
| | | local LSP/LSA | |
| | | | |
| | t0+60msec | C schedules SPF | |
| | | (100ms) | |
| | | | |
| | t0+67msec | C receives | |
| | | LSP/LSA from B | |
| | | | |
| | t0+70msec | C floods its | |
| | | local updated | |
| | | LSP/LSA | |
| | | | |
| | t0+87msec | | E receives LSP/LSA |
| | | | from C and schedules |
| | | | SPF (100ms) |
| | | | |
| | t0+117msec | | E floods LSP/LSA |
| | | | from C |
| | | | |
| | t0+160msec | C computes SPF | |
| | | | |
| | t0+165msec | C delays its | |
| | | RIB/FIB update | |
| | | (1 sec) | |
| | | | |
| | t0+193msec | | E computes SPF |
| | | | |
| | t0+199msec | | E starts updating |
| | | | its RIB/FIB |
| | | | |
| | t0+443msec | | E updates its |
| | | | RIB/FIB for D |
| | | | |
| | t0+470msec | | E convergence ends |
| | | | |
| | t0+1165msec | C starts | |
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| | | updating its | |
| | | RIB/FIB | |
| | | | |
| | t0+1255msec | C updates its | |
| | | RIB/FIB for D | |
| | | | |
| | t0+1340msec | C convergence | |
| | | ends | |
+-----------+-------------+------------------+----------------------+
Route computation event time scale
Similarly, upon B-C link down event, if LSP/LSA from B is received
before C detects the link failure, C will apply the route update
delay if the local detection is part of the same SPF run.
+-----------+-------------+------------------+----------------------+
| Network | Time | Router C events | Router E events |
| condition | | | |
+-----------+-------------+------------------+----------------------+
| S->D | | | |
| Traffic | | | |
| OK | | | |
| | | | |
| S->D | t0 | Link B-C fails | Link B-C fails |
| Traffic | | | |
| lost | | | |
| | | | |
| | t0+32msec | C receives | |
| | | LSP/LSA from B | |
| | | | |
| | t0+33msec | C schedules SPF | |
| | | (100ms) | |
| | | | |
| | t0+50msec | C detects the | |
| | | failure | |
| | | | |
| S->D | t0+55msec | C activates FRR | |
| Traffic | | | |
| OK | | | |
| | | | |
| | t0+55msec | C updates its | |
| | | local LSP/LSA | |
| | | | |
| | t0+70msec | C floods its | |
| | | local updated | |
| | | LSP/LSA | |
| | | | |
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| | t0+87msec | | E receives LSP/LSA |
| | | | from C and schedules |
| | | | SPF (100ms) |
| | | | |
| | t0+117msec | | E floods LSP/LSA |
| | | | from C |
| | | | |
| | t0+160msec | C computes SPF | |
| | | | |
| | t0+165msec | C delays its | |
| | | RIB/FIB update | |
| | | (1 sec) | |
| | | | |
| | t0+193msec | | E computes SPF |
| | | | |
| | t0+199msec | | E starts updating |
| | | | its RIB/FIB |
| | | | |
| | t0+443msec | | E updates its |
| | | | RIB/FIB for D |
| | | | |
| | t0+470msec | | E convergence ends |
| | | | |
| | t0+1165msec | C starts | |
| | | updating its | |
| | | RIB/FIB | |
| | | | |
| | t0+1255msec | C updates its | |
| | | RIB/FIB for D | |
| | | | |
| | t0+1340msec | C convergence | |
| | | ends | |
+-----------+-------------+------------------+----------------------+
Route computation event time scale
8.2. Local and remote event
The table below describes the events and associating timing that
happens on router C and E when link B-C goes down, in addition F-X
link will fail in the same time window. C will not apply the local
delay because a non local topology change is also received.
+-----------+------------+-----------------+------------------------+
| Network | Time | Router C events | Router E events |
| condition | | | |
+-----------+------------+-----------------+------------------------+
| S->D | | | |
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| Traffic | | | |
| OK | | | |
| | | | |
| S->D | t0 | Link B-C fails | Link B-C fails |
| Traffic | | | |
| lost | | | |
| | | | |
| | t0+20msec | C detects the | |
| | | failure | |
| | | | |
| | t0+36msec | Link F-X fails | Link F-X fails |
| | | | |
| S->D | t0+40msec | C activates FRR | |
| Traffic | | | |
| OK | | | |
| | | | |
| | t0+50msec | C updates its | |
| | | local LSP/LSA | |
| | | | |
| | t0+54msec | C receives | |
| | | LSP/LSA from F | |
| | | and floods it | |
| | | | |
| | t0+60msec | C schedules SPF | |
| | | (100ms) | |
| | | | |
| | t0+67msec | C receives | |
| | | LSP/LSA from B | |
| | | | |
| | t0+69msec | | E receives LSP/LSA |
| | | | from F, floods it and |
| | | | schedules SPF (100ms) |
| | | | |
| | t0+70msec | C floods its | |
| | | local updated | |
| | | LSP/LSA | |
| | | | |
| | t0+87msec | | E receives LSP/LSA |
| | | | from C |
| | | | |
| | t0+117msec | | E floods LSP/LSA from |
| | | | C |
| | | | |
| | t0+160msec | C computes SPF | |
| | | | |
| | t0+165msec | C starts | |
| | | updating its | |
| | | RIB/FIB (NO | |
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| | | DELAY) | |
| | | | |
| | t0+170msec | | E computes SPF |
| | | | |
| | t0+173msec | | E starts updating its |
| | | | RIB/FIB |
| | | | |
| S->D | t0+365msec | C updates its | |
| Traffic | | RIB/FIB for D | |
| lost | | | |
| | | | |
| S->D | t0+443msec | | E updates its RIB/FIB |
| Traffic | | | for D |
| OK | | | |
| | | | |
| | t0+450msec | C convergence | |
| | | ends | |
| | | | |
| | t0+470msec | | E convergence ends |
| | | | |
+-----------+------------+-----------------+------------------------+
Route computation event time scale
8.3. Aborting local delay
The table below describes the events and associating timing that
happens on router C and E when link B-C goes down, in addition F-X
link will fail during local delay run. C will first apply local
delay, but when the new event happens, it will fall back to the
standard convergence mechanism without delaying route insertion
anymore. In this example, we consider a ULOOP_DELAY_DOWN_TIMER
configured to 2 seconds.
+-----------+------------+-------------------+----------------------+
| Network | Time | Router C events | Router E events |
| condition | | | |
+-----------+------------+-------------------+----------------------+
| S->D | | | |
| Traffic | | | |
| OK | | | |
| | | | |
| S->D | t0 | Link B-C fails | Link B-C fails |
| Traffic | | | |
| lost | | | |
| | | | |
| | t0+20msec | C detects the | |
| | | failure | |
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| | | | |
| S->D | t0+40msec | C activates FRR | |
| Traffic | | | |
| OK | | | |
| | | | |
| | t0+50msec | C updates its | |
| | | local LSP/LSA | |
| | | | |
| | t0+60msec | C schedules SPF | |
| | | (100ms) | |
| | | | |
| | t0+67msec | C receives | |
| | | LSP/LSA from B | |
| | | | |
| | t0+70msec | C floods its | |
| | | local updated | |
| | | LSP/LSA | |
| | | | |
| | t0+87msec | | E receives LSP/LSA |
| | | | from C and schedules |
| | | | SPF (100ms) |
| | | | |
| | t0+117msec | | E floods LSP/LSA |
| | | | from C |
| | | | |
| | t0+160msec | C computes SPF | |
| | | | |
| | t0+165msec | C delays its | |
| | | RIB/FIB update (2 | |
| | | sec) | |
| | | | |
| | t0+193msec | | E computes SPF |
| | | | |
| | t0+199msec | | E starts updating |
| | | | its RIB/FIB |
| | | | |
| | t0+254msec | Link F-X fails | Link F-X fails |
| | | | |
| | t0+300msec | C receives | |
| | | LSP/LSA from F | |
| | | and floods it | |
| | | | |
| | t0+303msec | C schedules SPF | |
| | | (200ms) | |
| | | | |
| | t0+312msec | E receives | |
| | | LSP/LSA from F | |
| | | and floods it | |
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| | | | |
| | t0+313msec | E schedules SPF | |
| | | (200ms) | |
| | | | |
| | t0+502msec | C computes SPF | |
| | | | |
| | t0+505msec | C starts updating | |
| | | its RIB/FIB (NO | |
| | | DELAY) | |
| | | | |
| | t0+514msec | | E computes SPF |
| | | | |
| | t0+519msec | | E starts updating |
| | | | its RIB/FIB |
| | | | |
| S->D | t0+659msec | C updates its | |
| Traffic | | RIB/FIB for D | |
| lost | | | |
| | | | |
| S->D | t0+778msec | | E updates its |
| Traffic | | | RIB/FIB for D |
| OK | | | |
| | | | |
| | t0+781msec | C convergence | |
| | | ends | |
| | | | |
| | t0+810msec | | E convergence ends |
+-----------+------------+-------------------+----------------------+
Route computation event time scale
9. Comparison with other solutions
As stated in Section 3, our solution reuses some concepts already
introduced by other IETF proposals but tries to find a tradeoff
between efficiency and simplicity. This section tries to compare
behaviors of the solutions.
9.1. PLSN
PLSN ([I-D.ietf-rtgwg-microloop-analysis]) describes a mechanism
where each node in the network tries to avoid transient forwarding
loops upon a topology change by always keeping traffic on a loop-free
path for a defined duration (locked path to a safe neighbor). The
locked path may be the new primary nexthop, another neighbor, or the
old primary nexthop depending how the safety condition is satisfied.
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PLSN does not solve all transient forwarding loops (see
[I-D.ietf-rtgwg-microloop-analysis] Section 4 for more details).
Our solution reuses some concept of PLSN but in a more simple
fashion:
o PLSN has three different behaviors: keep using old nexthop, use
new primary nexthop if it is safe, or use another safe nexthop,
while our solution only have one: keep using the current nexthop
(old primary, or already activated FRR path).
o PLSN may cause some damage while using a safe nexthop which is not
the new primary nexthop in case the new safe nexthop does not
enough provide enough bandwidth (see [RFC7916]). Our solution may
not experience this issue as the service provider may have control
on the FRR path being used preventing network congestion.
o PLSN applies to all nodes in a network (remote or local changes),
while our mechanism applies only on the nodes connected to the
topology change.
9.2. OFIB
OFIB ([RFC6976]) describes a mechanism where the convergence of the
network upon a topology change is made ordered to prevent transient
forwarding loops. Each router in the network must deduce the failure
type from the LSA/LSP received and computes/applies a specific FIB
update timer based on the failure type and its rank in the network
considering the failure point as root.
This mechanism allows to solve all the transient forwarding loop in a
network at the price of introducing complexity in the convergence
process that may require a strong monitoring by the service provider.
Our solution reuses the OFIB concept but limits it to the first hop
that experiences the topology change. As demonstrated, our proposal
allows to solve all the local transient forwarding loops that
represents an high percentage of all the loops. Moreover limiting
the mechanism to one hop allows to keep the network-wide convergence
behavior.
10. Existing implementations
At this time, there are three different implementations of this
mechanism: CISCO IOS-XR, CISCO IOS-XE and Juniper JUNOS. The three
implementations have been tested in labs and demonstrated a good
behavior in term of local micro-loop avoidance. The feature has also
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been deployed in some live networks. No side effects have been
found.
11. Security Considerations
This document does not introduce any change in term of IGP security.
The operation is internal to the router. The local delay does not
increase the attack vector as an attacker could only trigger this
mechanism if he already has be ability to disable or enable an IGP
link. The local delay does not increase the negative consequences as
if an attacker has the ability to disable or enable an IGP link, it
can already harm the network by creating instability and harm the
traffic by creating forwarding packet loss and forwarding loss for
the traffic crossing that link.
12. Acknowledgements
We would like to thanks the authors of [RFC6976] for introducing the
concept of ordered convergence: Mike Shand, Stewart Bryant, Stefano
Previdi, and Olivier Bonaventure.
13. IANA Considerations
This document has no actions for IANA.
14. References
14.1. 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,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC5715] Shand, M. and S. Bryant, "A Framework for Loop-Free
Convergence", RFC 5715, DOI 10.17487/RFC5715, January
2010, <http://www.rfc-editor.org/info/rfc5715>.
14.2. Informative References
[I-D.ietf-rtgwg-backoff-algo]
Decraene, B., Litkowski, S., Gredler, H., Lindem, A.,
Francois, P., and C. Bowers, "SPF Back-off algorithm for
link state IGPs", draft-ietf-rtgwg-backoff-algo-05 (work
in progress), May 2017.
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[I-D.ietf-rtgwg-microloop-analysis]
Zinin, A., "Analysis and Minimization of Microloops in
Link-state Routing Protocols", draft-ietf-rtgwg-microloop-
analysis-01 (work in progress), October 2005.
[RFC3630] Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering
(TE) Extensions to OSPF Version 2", RFC 3630,
DOI 10.17487/RFC3630, September 2003,
<http://www.rfc-editor.org/info/rfc3630>.
[RFC6571] Filsfils, C., Ed., Francois, P., Ed., Shand, M., Decraene,
B., Uttaro, J., Leymann, N., and M. Horneffer, "Loop-Free
Alternate (LFA) Applicability in Service Provider (SP)
Networks", RFC 6571, DOI 10.17487/RFC6571, June 2012,
<http://www.rfc-editor.org/info/rfc6571>.
[RFC6976] Shand, M., Bryant, S., Previdi, S., Filsfils, C.,
Francois, P., and O. Bonaventure, "Framework for Loop-Free
Convergence Using the Ordered Forwarding Information Base
(oFIB) Approach", RFC 6976, DOI 10.17487/RFC6976, July
2013, <http://www.rfc-editor.org/info/rfc6976>.
[RFC7490] Bryant, S., Filsfils, C., Previdi, S., Shand, M., and N.
So, "Remote Loop-Free Alternate (LFA) Fast Reroute (FRR)",
RFC 7490, DOI 10.17487/RFC7490, April 2015,
<http://www.rfc-editor.org/info/rfc7490>.
[RFC7916] Litkowski, S., Ed., Decraene, B., Filsfils, C., Raza, K.,
Horneffer, M., and P. Sarkar, "Operational Management of
Loop-Free Alternates", RFC 7916, DOI 10.17487/RFC7916,
July 2016, <http://www.rfc-editor.org/info/rfc7916>.
Authors' Addresses
Stephane Litkowski
Orange
Email: stephane.litkowski@orange.com
Bruno Decraene
Orange
Email: bruno.decraene@orange.com
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Clarence Filsfils
Cisco Systems
Email: cfilsfil@cisco.com
Pierre Francois
Individual
Email: pfrpfr@gmail.com
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