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Versions: (draft-litkowski-rtgwg-uloop-delay)
00 01 02 03 04 05 06 07 08 09 RFC 8333
Routing Area Working Group S. Litkowski
Internet-Draft B. Decraene
Intended status: Standards Track Orange
Expires: December 5, 2016 C. Filsfils
P. Francois
Cisco Systems
June 3, 2016
Microloop prevention by introducing a local convergence delay
draft-ietf-rtgwg-uloop-delay-02
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 and the IGP convergence.
The proposed mechanism will be limited to 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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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 December 5, 2016.
Copyright Notice
Copyright (c) 2016 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
(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 . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Transient forwarding loops side effects . . . . . . . . . . . 3
2.1. Fast reroute unefficiency . . . . . . . . . . . . . . . . 4
2.2. Network congestion . . . . . . . . . . . . . . . . . . . 6
3. Overview of the solution . . . . . . . . . . . . . . . . . . 7
4. Specification . . . . . . . . . . . . . . . . . . . . . . . . 7
4.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . 7
4.2. Current IGP reactions . . . . . . . . . . . . . . . . . . 7
4.3. Local events . . . . . . . . . . . . . . . . . . . . . . 8
4.4. Local delay for link down . . . . . . . . . . . . . . . . 8
5. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 9
5.1. Applicable case : local loops . . . . . . . . . . . . . . 9
5.2. Non applicable case : remote loops . . . . . . . . . . . 9
6. Simulations . . . . . . . . . . . . . . . . . . . . . . . . . 10
7. Deployment considerations . . . . . . . . . . . . . . . . . . 11
8. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 11
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 . . . . . . . . . . . . . . . . . . . 20
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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 local to the failure; based on
network analysis, these are a significant portion of the micro-
forwarding loops. A simple and easily deployable solution to these
local micro-loops is critical because these local loops cause traffic
loss after an advanced 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 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 damage for the network.
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2.1. Fast reroute unefficiency
D
1 |
| 1
A ------ B
| | ^
10 | | 5 | T
| | |
E--------C
| 1
1 |
S
Figure 2 - RSVPTE FRR case
In figure 2, a RSVP-TE tunnel T, provisionned on C and terminating on
B, is used to protect against C-B link failure (IGP shortcut
activated on C). Primary path of T is C->B and FRR is activated on T
providing a FRR bypass or detour using path C->E->A->B. On C,
nexthop to D is tunnel T thanks to IGP shortcut. When C-B link fails
:
1. C detects the failure, and updates the tunnel path using
preprogrammed FRR path, traffic path from S to D is :
S->E->C->E->A->B->A->D .
2. In parallel, on router C, both IGP convergence and TE tunnel
convergence (tunnel path recomputation) are occuring :
* T path is recomputed : C->E->A->B
* IGP path to D is recomputed : C->E->A->D
3. On C, tail-end of the TE tunnel (router B) is no more on SPT to
D, so C does not encapsulate anymore the traffic to D using the
tunnel T and update forwarding entry to D using 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 | | | |
| 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+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 | |
| | | | |
| S->D | t0+443msec | | E updates its RIB/FIB |
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| 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 ...). Fast-reroute
is working perfectly but ensures 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 IGP convergence may permit to
keep fast-reroute path until neighbor has converged and preserve
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 link S-D fails,
a transient forwarding loop may appear between S and B for
destination D. The traffic on S-B link will constantly increase due
to the looping traffic to D. Depending on TTL of packets, traffic
rate destinated to D and bandwidth of link, the S-B link may be
congestioned in few hundreds of milliseconds and will stay overloaded
until the loop is solved.
Congestion introduced by transient forwarding loops are problematic
as they are impacting traffic that is not directly concerned by the
failing network component. In our example, the congestion of S-B
link will impact customer traffic that is not directly concerned by
the failure : e.g. A to B, F to B, E to B. Class of services may be
implemented to mitigate the congestion but some traffic not directly
concerned by the failure would still be dropped as a router is not
able to identify looped traffic from normal traffic.
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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.
This ordered convergence, is similar to the ordered FIB proposed
defined in [RFC6976], but limited to only 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 limitation.
4. Specification
4.1. Definitions
This document will refer to the following existing IGP timers:
o LSP_GEN_TIMER: to batch multiple local events in one single local
LSP update. It is often associated with damping mechanism to
slowdown reactions by incrementing the timer when multiple
consecutive events are detected.
o SPF_TIMER: to batch multiple events in one single computation. It
is often associated with damping mechanism to slowdown reactions
by incrementing the timer when the IGP is instable.
This document introduces the following a new timer :
o ULOOP_DELAY_DOWN_TIMER: slowdown the local node convergence in
case of link down events.
4.2. Current IGP reactions
Upon a change of status on an adjacency/link, the existing behavior
of the router advertising the event is the following:
1. UP/Down event is notified to IGP.
2. IGP processes the notification and postpones the reaction in
LSP_GEN_TIMER msec.
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3. Upon LSP_GEN_TIMER expiration, IGP updates its LSP/LSA and floods
it.
4. SPF is scheduled in SPF_TIMER msec.
5. Upon SPF_TIMER expiration, SPF is computed and RIB/FIB are
updated.
4.3. Local events
The mechanisms described in this document assume 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). There are three types of single failures: local link,
local node, and remote failure.
Example :
+--- E ----+--------+
| | |
A ---- B -------- C ------ D
Let 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_TIMER
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.
An implementation SHOULD implement a logic to correlate protocol
messages (LSP/LSA) received during SPF scheduling and topology
changes as multiple protocol messages may describe the same topology
change. 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
event is a single local link failure, then the router MAY use the
mechanism described in this document, otherwise standard IP
convergence MUST be used.
4.4. Local delay for link down
Upon an adjacency/link down event, this document introduces a change
in step 5 in order to delay the local convergence compared to the
network wide convergence: the node SHOULD delay the forwarding entry
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updates by ULOOP_DELAY_DOWN_TIMER. Such delay SHOULD only be
introduced if all the LSDB modifications processed are only reporting
down local events . Note that determining that all topological
change are only local down events requires analyzing all modified
LSP/LSA as a local link or node failure will typically be notified by
multiple nodes. 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 non urgent convergence, such as for administrative de-activation
(maintenance) or when the traffic is Fast ReRouted.
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 CE 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.
By implementing the mechanism defined in this document on C, when the
CE link fails, C delays the update of his 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, forwarding
entry to F is updated. There is no transient forwarding loop on the
link CD.
5.2. Non applicable case : remote loops
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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 CF 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 CF fails, C delays the update of his forwarding entry to K,
letting time for D to converge. When the timer expires on C,
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 occuring.
6. Simulations
Simulations have been run on multiple service provider topologies.
So far, only link down event have been tested.
+----------+------+
| 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
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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 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 Limit FRR efficiency : even if FRR is activated in 50msec, as soon
as PLR has converged, traffic may be affected by a transient loop.
o It 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 address 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
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 :
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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 FRR 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 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 | | | |
| | | | |
| 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) | |
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| | | | |
| | 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 | |
| | | 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.
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+-----------+-------------+------------------+----------------------+
| 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 | |
| | | | |
| | 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 |
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| | | | 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 | | | |
| 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 | | | |
| | | | |
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| | 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 | |
| | | 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 | |
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| | | 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 fallback 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 | |
| | | | |
| 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 | |
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| | | 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 | |
| | | | |
| | 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 | |
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| 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 a 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 satisified.
PLSN does not solve all transient forwarding loops (see
[I-D.ietf-rtgwg-microloop-analysis] Section 4 for more details).
Our solution reuse some concept of PLSN but in a more simple fashion
:
o PLSN has 3 different behavior : keep using old nexthop, use new
primary nexthop if 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
[I-D.ietf-rtgwg-lfa-manageability]). Our solution may not
experience this issue as the service provider may have control on
the FRR path being used preventing network congestion.
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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 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 compute/apply a specific FIB
update timer based on the failure type and its rank in the network
considering the failure point as root.
This mechanism permit to solve all the transient forwarding loop in a
network at the price of introducing complexity in the convergence
process that may require strong monitoring by the service provider.
Our solution reuses the OFIB concept but limits it to the first hop
that experience the topology change. As demonstrated, our proposal
permits to solve all the local transient forwarding loops that
represents a high percentage of all the loops. Moreover limiting the
mechanism to one hop permit to keep the network-wide convergence
behavior.
10. Existing implementations
At this time, there is 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. No side effects have
been found.
11. Security Considerations
This document does not introduce 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.
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12. Acknowledgements
We wish 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-lfa-manageability]
Litkowski, S., Decraene, B., Filsfils, C., Raza, K., and
M. Horneffer, "Operational management of Loop Free
Alternates", draft-ietf-rtgwg-lfa-manageability-11 (work
in progress), June 2015.
[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>.
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[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>.
Authors' Addresses
Stephane Litkowski
Orange
Email: stephane.litkowski@orange.com
Bruno Decraene
Orange
Email: bruno.decraene@orange.com
Clarence Filsfils
Cisco Systems
Email: cfilsfil@cisco.com
Pierre Francois
Cisco Systems
Email: pifranco@cisco.com
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