Internet Engineering Task Force (IETF) S. Litkowski
Request for Comments: 8333 B. Decraene
Category: Standards Track Orange
ISSN: 2070-1721 C. Filsfils
Cisco Systems
P. Francois
Individual Contributor
March 2018
Micro-loop Prevention by Introducing a Local Convergence Delay
Abstract
This document describes a mechanism for link-state routing protocols
that prevents local transient forwarding loops in case of link
failure. This mechanism proposes a two-step convergence by
introducing a delay between the convergence of the node adjacent to
the topology change and the network-wide convergence.
Because this mechanism delays the IGP convergence, it may only be
used for planned maintenance or when Fast Reroute (FRR) protects the
traffic during the time between the link failure and the IGP
convergence.
The mechanism is limited to the link-down event in order to keep the
mechanism simple.
Simulations using real network topologies have been performed and
show that local loops are a significant portion (>50%) of the total
forwarding loops.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8333.
Litkowski, et al. Standards Track [Page 1]RFC 8333 Micro-loop Prevention by Local Delay March 2018
Copyright Notice
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document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Litkowski, et al. Standards Track [Page 2]
RFC 8333 Micro-loop Prevention by Local Delay March 2018
Table of Contents
1. Introduction ....................................................4
2. Terminology .....................................................4
2.1. Acronyms ...................................................4
2.2. Requirements Language ......................................5
3. Side Effects of Transient Forwarding Loops ......................5
3.1. FRR Inefficiency ...........................................5
3.2. Network Congestion .........................................8
4. Overview of the Solution ........................................9
5. Specification ...................................................9
5.1. Definitions ................................................9
5.2. Regular IGP Reaction ......................................10
5.3. Local Events ..............................................10
5.4. Local Delay for Link-Down Events ..........................11
6. Applicability ..................................................11
6.1. Applicable Case: Local Loops ..............................12
6.2. Non-applicable Case: Remote Loops .........................12
7. Simulations ....................................................13
8. Deployment Considerations ......................................14
9. Examples .......................................................15
9.1. Local Link-Down Event .....................................15
9.2. Local and Remote Event ....................................19
9.3. Aborting Local Delay ......................................21
10. Comparison with Other Solutions ...............................23
10.1. PLSN .....................................................23
10.2. oFIB .....................................................24
11. IANA Considerations ...........................................24
12. Security Considerations .......................................24
13. References ....................................................25
13.1. Normative References .....................................25
13.2. Informative References ...................................25
Acknowledgements ..................................................26
Authors' Addresses ................................................26
Litkowski, et al. Standards Track [Page 3]RFC 8333 Micro-loop Prevention by Local Delay March 2018
1. Introduction
Micro-loops and some potential solutions are described in [RFC5715].
This document describes a simple targeted mechanism that prevents
micro-loops that are local to the failure. Based on network
analysis, local micro-loops make up a significant portion of the
micro-loops. A simple and easily deployable solution for these local
micro-loops is critical because these local loops cause some traffic
loss after an FRR alternate has been used (see Section 3.1).
Consider the case in Figure 1 where S does not have an LFA (Loop-Free
Alternate) to protect its traffic to D when the S-D link fails. 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 FRR technique used,
when S converges to the new topology, it will send its traffic to a
neighbor that is not loop-free and will thus cause a local micro-
loop. The deployment of advanced FRR techniques motivates this
simple router-local mechanism to solve this targeted problem. This
solution can work with the various techniques described in [RFC5715].
D ------ C
| |
| | 5
| |
S ------ B
Figure 1
In Figure 1, all links have a metric of 1 except the B-C link, which
has a metric of 5. When the S-D link fails, a transient forwarding
loop may appear between S and B if S updates its forwarding entry to
D before B does.
2. Terminology
2.1. Acronyms
FIB: Forwarding Information Base
FRR: Fast Reroute
IGP: Interior Gateway Protocol
LFA: Loop-Free Alternate
LSA: Link State Advertisement
Litkowski, et al. Standards Track [Page 4]RFC 8333 Micro-loop Prevention by Local Delay March 2018
LSP: Link State Packet
MRT: Maximally Redundant Tree
oFIB: Ordered FIB
PLR: Point of Local Repair
PLSN: Path Locking via Safe Neighbors
RIB: Routing Information Base
RLFA: Remote Loop-Free Alternate
SPF: Shortest Path First
TTL: Time to Live
2.2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Side Effects of Transient Forwarding Loops
Even if they are very limited in duration, transient forwarding loops
may cause significant network damage.
3.1. FRR Inefficiency
In Figure 2, we consider an IP/LDP routed network.
D
1 |
| 1
A ------ B
| | ^
10 | | 5 | T
| | |
E--------C
| 1
1 |
S
Figure 2
Litkowski, et al. Standards Track [Page 5]RFC 8333 Micro-loop Prevention by Local Delay March 2018
An RSVP-TE tunnel T, provisioned on C and terminating on B, is used
to protect the traffic against C-B link failure (the IGP shortcut
feature, defined in [RFC3906], 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 router C, the next hop to D is the
tunnel T, thanks to the IGP shortcut. When the C-B link fails:
1. C detects the failure and updates the tunnel path using a
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 longer on
the shortest-path tree (SPT) to D, so C does not continue to
encapsulate the traffic to D using the tunnel T and updates its
forwarding entry to D using the next-hop 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.
Table 1 describes a theoretical sequence of events happening when the
B-C link fails. This theoretical sequence of events should only be
read as an example.
+------------+--------+---------------------+-----------------------+
| 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+20 | C detects the | |
| | ms | failure | |
| | | | |
Litkowski, et al. Standards Track [Page 6]RFC 8333 Micro-loop Prevention by Local Delay March 2018
| S->D | t0+40 | C activates FRR | |
| Traffic OK | ms | | |
| | | | |
| | t0+50 | C updates its local | |
| | ms | LSP/LSA | |
| | | | |
| | t0+60 | C floods its local | |
| | ms | updated LSP/LSA | |
| | | | |
| | t0+62 | C schedules SPF | |
| | ms | (100 ms) | |
| | | | |
| | t0+87 | | E receives LSP/LSA |
| | ms | | from C and floods it |
| | | | |
| | t0+92 | | E schedules SPF (100 |
| | ms | | ms) |
| | | | |
| | t0+163 | C computes SPF | |
| | ms | | |
| | | | |
| | t0+165 | C starts updating | |
| | ms | its RIB/FIB | |
| | | | |
| | t0+193 | | E computes SPF |
| | ms | | |
| | | | |
| | t0+199 | | E starts updating its |
| | ms | | RIB/FIB |
| | | | |
| S->D | t0+255 | C updates its | |
| Traffic | ms | RIB/FIB for D | |
| lost | | | |
| | | | |
| | t0+340 | C convergence ends | |
| | ms | | |
| | | | |
| S->D | t0+443 | | E updates its RIB/FIB |
| Traffic OK | ms | | for D |
| | | | |
| | t0+470 | | E convergence ends |
| | ms | | |
+------------+--------+---------------------+-----------------------+
Table 1
Litkowski, et al. Standards Track [Page 7]RFC 8333 Micro-loop Prevention by Local Delay March 2018
The issue described here is completely independent of the FRR
mechanism involved (e.g., TE FRR, LFA/RLFA, MRT, etc.) when the
primary path uses hop-by-hop routing. The protection enabled by FRR
works perfectly but only ensures protection until the PLR has
converged (as soon as the PLR has converged, it replaces its FRR path
with a new primary path). When implementing FRR, a service provider
wants to guarantee a very limited loss of connectivity time. The
example described in this section 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
(1) may allow the FRR path to be kept until the neighbors have
converged and (2) preserves the customer traffic.
3.2. Network Congestion
In Figure 3, when the S-D link 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 destined to D,
and the bandwidth of the link, the S-B link may become congested in a
few hundreds of milliseconds and will stay congested until the loop
is eliminated.
1
D ------ C
| |
1 | | 5
| |
A -- S ------ B
/ | 1
F E
Figure 3
The congestion introduced by transient forwarding loops is
problematic as it can affect traffic that is not directly affected by
the failing network component. In Figure 3, the congestion of the
S-B link will impact some customer traffic that is not directly
affected by the failure, e.g., traffic from A to B, F to B, and E to
B. Class of service may mitigate the congestion for some traffic.
However, some traffic not directly affected by the failure will still
be dropped as a router is not able to distinguish the looping traffic
from the normally forwarded traffic.
Litkowski, et al. Standards Track [Page 8]RFC 8333 Micro-loop Prevention by Local Delay March 2018
4. Overview of the Solution
This document defines a two-step convergence initiated by the router
detecting a failure and advertising the topological change in the
IGP. This introduces a delay between network-wide convergence and
the convergence of the local router.
The solution described in this document is limited to local link-down
events in order to keep the solution simple.
This ordered convergence is similar to the ordered FIB (oFIB)
approach defined in [RFC6976], but it is limited to only a "one-hop"
distance. As a consequence, it is more simple and becomes a local-
only feature that does not require interoperability. This benefit
comes with the limitation of eliminating transient forwarding loops
involving the local router only. The mechanism also reuses some
concepts described in [PLSN].
5. Specification
5.1. Definitions
This document refers to the following existing IGP timers. These
timers may be standardized or implemented as a vendor-specific local
feature.
o LSP_GEN_TIMER: The delay between the consecutive generation of two
local LSPs/LSAs. From an operational point of view, this delay is
usually tuned to batch multiple local events in a single local
LSP/LSA update. In IS-IS, this timer is defined as
minimumLSPGenerationInterval [ISO10589]. In OSPF version 2, this
timer is defined as MinLSInterval [RFC2328]. It is often
associated with a vendor-specific 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, [BACKOFF] defines a standard SPF delay
algorithm.
Litkowski, et al. Standards Track [Page 9]RFC 8333 Micro-loop Prevention by Local Delay March 2018
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.
5.2. Regular IGP Reaction
When the status of an adjacency or link changes, the regular IGP
convergence behavior of the router advertising the event involves the
following main steps:
1. IGP is notified of the up/down event.
2. The IGP processes the notification and postpones the reaction for
LSP_GEN_TIMER ms.
3. Upon LSP_GEN_TIMER expiration, the IGP updates its LSP/LSA and
floods it.
4. The SPF computation is scheduled in SPF_DELAY ms.
5. Upon SPF_DELAY timer expiration, the SPF is computed, and then
the RIB and FIB are updated.
5.3. 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
regular IP convergence must be applied (as described in Section 5.2).
To determine if the mechanism is applicable or not, an implementation
SHOULD implement logic to correlate the protocol messages (LSP/LSA)
received during the SPF scheduling period in order to determine the
topology changes that occurred. 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 left to the implementation.
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 regular
IP convergence MUST be used.
Litkowski, et al. Standards Track [Page 10]RFC 8333 Micro-loop Prevention by Local Delay March 2018
In Figure 4, 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 LSPs/LSAs.
During the SPF_DELAY period, B and C learn all the LSPs/LSAs to
consider. B sees that C is flooding an advertisement that indicates
that a link is down, and B is the other end of that link. B
determines 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.
+--- E ----+--------+
| | |
A ---- B -------- C ------ D
Figure 4
5.4. Local Delay for Link-Down Events
This document introduces a change in step 5 (see list in Section 5.2)
so that, upon an adjacency or link-down event, the local convergence
is delayed compared to the network-wide convergence. The new step 5
is described below:
5. Upon SPF_DELAY timer expiration, the SPF is computed. If the
condition of a single local link-down event has been met, then an
update of the RIB and the FIB MUST be delayed for
ULOOP_DELAY_DOWN_TIMER ms. Otherwise, the RIB and FIB SHOULD be
updated immediately.
If a new convergence occurs while ULOOP_DELAY_DOWN_TIMER is running,
ULOOP_DELAY_DOWN_TIMER is stopped, and the RIB/FIB SHOULD be updated
as part of the new convergence event.
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 administrative deactivation
(maintenance) or when the traffic is protected by FRR.
6. Applicability
As previously stated, this mechanism only avoids the forwarding loops
on the links between the node local to the failure and its neighbors.
Forwarding loops may still occur on other links.
Litkowski, et al. Standards Track [Page 11]RFC 8333 Micro-loop Prevention by Local Delay March 2018
6.1. Applicable Case: Local Loops
In Figure 5, let us consider the traffic from G to F. The primary
path is G->D->C->E->F. When the link C-E fails, if C updates its
forwarding entry for F before D, a transient loop occurs. This is
sub-optimal as it breaks C's FRR forwarding even though upstream
routers are still forwarding the traffic to C.
A ------ B ----- E
| / |
| / |
G---D------------C F
All the links have a metric of 1
Figure 5
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 allow some time for D to converge. FRR on C keeps
protecting the traffic during this period. When
ULOOP_DELAY_DOWN_TIMER expires on C, its forwarding entry to F is
updated. There is no transient forwarding loop on the link C-D.
6.2. Non-applicable Case: Remote Loops
In Figure 6, 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.
A ------ B ----- E --- H
| |
| |
G---D--------C ------F --- J ---- K
All the links have a metric of 1 except B-E=15
Figure 6
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,
allowing time for D to converge. When ULOOP_DELAY_DOWN_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
Litkowski, et al. Standards Track [Page 12]RFC 8333 Micro-loop Prevention by Local Delay March 2018
some cases -- such as when the nodes update their FIB in the order C,
A, D because the router A is quicker than D to converge -- the
mechanism may still avoid the forwarding loop that would have
otherwise occurred.
7. Simulations
Simulations have been run on multiple service-provider topologies.
We evaluated the efficiency of the mechanism on eight different
service-provider topologies (different network size and design).
Table 2 displays the gain for each topology.
+----------+------+
| Topology | Gain |
+----------+------+
| T1 | 71% |
| T2 | 81% |
| T3 | 62% |
| T4 | 50% |
| T5 | 70% |
| T6 | 70% |
| T7 | 59% |
| T8 | 77% |
+----------+------+
Table 2
We evaluated the gain as follows:
o We considered a tuple (link A-B, destination D, PLR S, backup
next-hop 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
its neighbors N.
o We evaluated the number of potential loop tuples in normal
conditions.
o We evaluated the number of potential loop tuples using the same
topological input but taking into account that S converges after
N.
o The gain is the relative number of loops (both remote and local)
we succeed in suppressing.
Litkowski, et al. Standards Track [Page 13]RFC 8333 Micro-loop Prevention by Local Delay March 2018
For topology 1, implementing the local delay prevented 71% of the
transient forwarding loops created by the failure of any link. The
analysis shows that all local loops are prevented and only remote
loops remain.
8. Deployment Considerations
Transient forwarding loops have the following drawbacks:
o They limit FRR efficiency. Even if FRR is activated within 50 ms,
as soon as the PLR has converged, the traffic may be affected by a
transient loop.
o They may impact traffic not directly affected by the failure (due
to link congestion).
The local delay mechanism 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.
The ULOOP_DELAY_DOWN_TIMER value should be set according to the
maximum IGP convergence time observed in the network (usually
observed in the slowest node).
This mechanism is limited to link-down events. When a link goes
down, it eventually goes back up. As a consequence, with this
mechanism deployed, only the link-down event will be protected
against transient forwarding loops while the link-up event will not.
If the operator wants to limit the impact of transient forwarding
loops during the link-up event, it should make sure to use specific
procedures to bring the link back online. As examples, the operator
can decide to put the link back online outside of business hours, or
it can use some incremental metric changes to prevent loops (as
proposed in [RFC5715]).
Litkowski, et al. Standards Track [Page 14]RFC 8333 Micro-loop Prevention by Local Delay March 2018
9. Examples
We consider the following figure for the examples in this section:
D
1 | F----X
| 1 |
A ------ B
| |
10 | | 5
| |
E--------C
| 1
1 |
S
Figure 7
The network above is considered to have a convergence time of about 1
second, so ULOOP_DELAY_DOWN_TIMER will be adjusted to this value. We
also consider that FRR is running on each node.
9.1. Local Link-Down Event
Table 3 describes the events and their timing on routers C and E when
the link B-C goes down. It is based on a theoretical sequence of
events that should only been read as an example. As C detects a
single local event corresponding to a link-down event (its LSP + LSP
from B received), it applies the local delay down behavior, and no
micro-loop is formed.
Litkowski, et al. Standards Track [Page 15]RFC 8333 Micro-loop Prevention by Local Delay March 2018
+------------+---------+---------------------+----------------------+
| 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+20 | C detects the | |
| | ms | failure | |
| | | | |
| S->D | t0+40 | C activates FRR | |
| Traffic OK | ms | | |
| | | | |
| | t0+50 | C updates its local | |
| | ms | LSP/LSA | |
| | | | |
| | t0+53 | C floods its local | |
| | ms | updated LSP/LSA | |
| | | | |
| | t0+60 | C schedules SPF | |
| | ms | (100 ms) | |
| | | | |
| | t0+67 | C receives LSP/LSA | |
| | ms | from B and floods | |
| | | it | |
| | | | |
| | t0+87 | | E receives LSP/LSA |
| | ms | | from C and floods it |
| | | | |
| | t0+90 | | E schedules SPF (100 |
| | ms | | ms) |
| | | | |
| | t0+161 | C computes SPF | |
| | ms | | |
| | | | |
| | t0+165 | C delays its | |
| | ms | RIB/FIB update (1 | |
| | | sec) | |
| | | | |
| | t0+193 | | E computes SPF |
| | ms | | |
| | | | |
| | t0+199 | | E starts updating |
| | ms | | its RIB/FIB |
Litkowski, et al. Standards Track [Page 16]
RFC 8333 Micro-loop Prevention by Local Delay March 2018
| | | | |
| | t0+443 | | E updates its |
| | ms | | RIB/FIB for D |
| | | | |
| | t0+470 | | E convergence ends |
| | ms | | |
| | | | |
| | t0+1165 | C starts updating | |
| | ms | its RIB/FIB | |
| | | | |
| | t0+1255 | C updates its | |
| | ms | RIB/FIB for D | |
| | | | |
| | t0+1340 | C convergence ends | |
| | ms | | |
+------------+---------+---------------------+----------------------+
Table 3
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. Table 4
describes the associated theoretical sequence of events. It should
only been read as an example.
+------------+---------+---------------------+----------------------+
| 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+32 | C receives LSP/LSA | |
| | ms | from B and floods | |
| | | it | |
| | | | |
| | t0+33 | C schedules SPF | |
| | ms | (100 ms) | |
| | | | |
| | t0+50 | C detects the | |
| | ms | failure | |
| | | | |
Litkowski, et al. Standards Track [Page 17]RFC 8333 Micro-loop Prevention by Local Delay March 2018
| S->D | t0+55 | C activates FRR | |
| Traffic OK | ms | | |
| | | | |
| | t0+55 | C updates its local | |
| | ms | LSP/LSA | |
| | | | |
| | t0+70 | C floods its local | |
| | ms | updated LSP/LSA | |
| | | | |
| | t0+87 | | E receives LSP/LSA |
| | ms | | from C and floods it |
| | | | |
| | t0+90 | | E schedules SPF (100 |
| | ms | | ms) |
| | | | |
| | t0+135 | C computes SPF | |
| | ms | | |
| | | | |
| | t0+140 | C delays its | |
| | ms | RIB/FIB update (1 | |
| | | sec) | |
| | | | |
| | t0+193 | | E computes SPF |
| | ms | | |
| | | | |
| | t0+199 | | E starts updating |
| | ms | | its RIB/FIB |
| | | | |
| | t0+443 | | E updates its |
| | ms | | RIB/FIB for D |
| | | | |
| | t0+470 | | E convergence ends |
| | ms | | |
| | | | |
| | t0+1145 | C starts updating | |
| | ms | its RIB/FIB | |
| | | | |
| | t0+1255 | C updates its | |
| | ms | RIB/FIB for D | |
| | | | |
| | t0+1340 | C convergence ends | |
| | ms | | |
+------------+---------+---------------------+----------------------+
Table 4
Litkowski, et al. Standards Track [Page 18]RFC 8333 Micro-loop Prevention by Local Delay March 2018
9.2. Local and Remote Event
Table 5 describes the events and their timing on router C and E when
the link B-C goes down and when the link F-X fails in the same time
window. C will not apply the local delay because a non-local
topology change is also received. Table 5 is based on a theoretical
sequence of events that should only been read as an example.
+-----------+--------+-------------------+--------------------------+
| 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+20 | C detects the | |
| | ms | failure | |
| | | | |
| | t0+36 | Link F-X fails | Link F-X fails |
| | ms | | |
| | | | |
| S->D | t0+40 | C activates FRR | |
| Traffic | ms | | |
| OK | | | |
| | | | |
| | t0+50 | C updates its | |
| | ms | local LSP/LSA | |
| | | | |
| | t0+54 | C receives | |
| | ms | LSP/LSA from F | |
| | | and floods it | |
| | | | |
| | t0+60 | C schedules SPF | |
| | ms | (100 ms) | |
| | | | |
| | t0+67 | C receives | |
| | ms | LSP/LSA from B | |
| | | and floods it | |
| | | | |
| | t0+69 | | E receives LSP/LSA from |
| | ms | | F, floods it and |
| | | | schedules SPF (100 ms) |
| | | | |
Litkowski, et al. Standards Track [Page 19]
RFC 8333 Micro-loop Prevention by Local Delay March 2018
| | t0+70 | C floods its | |
| | ms | local updated | |
| | | LSP/LSA | |
| | | | |
| | t0+87 | | E receives LSP/LSA from |
| | ms | | C |
| | | | |
| | t0+117 | | E floods LSP/LSA from C |
| | ms | | |
| | | | |
| | t0+160 | C computes SPF | |
| | ms | | |
| | | | |
| | t0+165 | C starts updating | |
| | ms | its RIB/FIB (NO | |
| | | DELAY) | |
| | | | |
| | t0+170 | | E computes SPF |
| | ms | | |
| | | | |
| | t0+173 | | E starts updating its |
| | ms | | RIB/FIB |
| | | | |
| S->D | t0+365 | C updates its | |
| Traffic | ms | RIB/FIB for D | |
| lost | | | |
| | | | |
| S->D | t0+443 | | E updates its RIB/FIB |
| Traffic | ms | | for D |
| OK | | | |
| | | | |
| | t0+450 | C convergence | |
| | ms | ends | |
| | | | |
| | t0+470 | | E convergence ends |
| | ms | | |
| | | | |
+-----------+--------+-------------------+--------------------------+
Table 5
Litkowski, et al. Standards Track [Page 20]RFC 8333 Micro-loop Prevention by Local Delay March 2018
9.3. Aborting Local Delay
Table 6 describes the events and their timing on routers C and E when
the link B-C goes down. In addition, we consider what happens when
the F-X link fails during local delay of the FIB update. C will
first apply the local delay, but when the new event happens, it will
fall back to the standard convergence mechanism without further
delaying route insertion. In this example, we consider a
ULOOP_DELAY_DOWN_TIMER configured to 2 seconds. Table 6 is based on
a theoretical sequence of events that should only been read as an
example.
+------------+--------+----------------------+----------------------+
| 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+20 | C detects the | |
| | ms | failure | |
| | | | |
| S->D | t0+40 | C activates FRR | |
| Traffic OK | ms | | |
| | | | |
| | t0+50 | C updates its local | |
| | ms | LSP/LSA | |
| | | | |
| | t0+55 | C floods its local | |
| | ms | updated LSP/LSA | |
| | | | |
| | t0+57 | C schedules SPF (100 | |
| | ms | ms) | |
| | | | |
| | t0+67 | C receives LSP/LSA | |
| | ms | from B and floods it | |
| | | | |
| | t0+87 | | E receives LSP/LSA |
| | ms | | from C and floods it |
| | | | |
| | t0+90 | | E schedules SPF (100 |
| | ms | | ms) |
| | | | |
Litkowski, et al. Standards Track [Page 21]
RFC 8333 Micro-loop Prevention by Local Delay March 2018
| | t0+160 | C computes SPF | |
| | ms | | |
| | | | |
| | t0+165 | C delays its RIB/FIB | |
| | ms | update (2 sec) | |
| | | | |
| | t0+193 | | E computes SPF |
| | ms | | |
| | | | |
| | t0+199 | | E starts updating |
| | ms | | its RIB/FIB |
| | | | |
| | t0+254 | Link F-X fails | Link F-X fails |
| | ms | | |
| | | | |
| | t0+300 | C receives LSP/LSA | |
| | ms | from F and floods it | |
| | | | |
| | t0+303 | C schedules SPF (200 | |
| | ms | ms) | |
| | | | |
| | t0+312 | E receives LSP/LSA | |
| | ms | from F and floods it | |
| | | | |
| | t0+313 | E schedules SPF (200 | |
| | ms | ms) | |
| | | | |
| | t0+502 | C computes SPF | |
| | ms | | |
| | | | |
| | t0+505 | C starts updating | |
| | ms | its RIB/FIB (NO | |
| | | DELAY) | |
| | | | |
| | t0+514 | | E computes SPF |
| | ms | | |
| | | | |
| | t0+519 | | E starts updating |
| | ms | | its RIB/FIB |
| | | | |
| S->D | t0+659 | C updates its | |
| Traffic | ms | RIB/FIB for D | |
| lost | | | |
| | | | |
Litkowski, et al. Standards Track [Page 22]
RFC 8333 Micro-loop Prevention by Local Delay March 2018
| S->D | t0+778 | | E updates its |
| Traffic OK | ms | | RIB/FIB for D |
| | | | |
| | t0+781 | C convergence ends | |
| | ms | | |
| | | | |
| | t0+810 | | E convergence ends |
| | ms | | |
+------------+--------+----------------------+----------------------+
Table 6
10. Comparison with Other Solutions
As stated in Section 4, the local delay solution reuses some concepts
already introduced by other IETF proposals but tries to find a trade-
off between efficiency and simplicity. This section tries to compare
behaviors of the solutions.
10.1. PLSN
PLSN [PLSN] 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 next hop, another neighbor, or the old primary next hop
depending on how the safety condition is satisfied.
PLSN does not solve all transient forwarding loops (see Section 4 of
[PLSN] for more details).
The solution defined in this document reuses some concepts of PLSN
but in a more simple fashion:
o PLSN has three different behaviors: (1) keep using the old next
hop, (2) use the new primary next hop if it is safe, or (3) use
another safe next hop. The local delay solution, however, only
has one: keep using the current next hop (i.e., the old primary
next hop or an already-activated FRR path).
o PLSN may cause some damage while using a safe next hop that is not
the new primary next hop if the new safe next hop does not provide
enough bandwidth (see [RFC7916]). The solution defined in this
document may not experience this issue as the service provider may
have control on the FRR path being used, preventing network
congestion.
Litkowski, et al. Standards Track [Page 23]RFC 8333 Micro-loop Prevention by Local Delay March 2018
o PLSN applies to all nodes in a network (remote or local changes),
while the mechanism defined in this document applies only to the
nodes connected to the topology change.
10.2. oFIB
oFIB [RFC6976] describes a mechanism where the convergence of the
network upon a topology change is ordered in order to prevent
transient forwarding loops. Each router in the network deduces 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.
The oFIB mechanism solves all the transient forwarding loops in a
network at the price of introducing complexity in the convergence
process that may require careful monitoring by the service provider.
The solution defined in this document reuses the oFIB concept but
limits it to the first hop that experiences the topology change. As
demonstrated, the mechanism defined in this document allows all the
local transient forwarding loops to be solved; these represent a high
percentage of all the loops. Moreover, limiting to one hop allows
network-wide convergence behavior to be kept.
11. IANA Considerations
This document has no IANA actions.
12. Security Considerations
This document does not introduce any change in terms of IGP security.
The operation is internal to the router. The local delay does not
increase the number of attack vectors as an attacker could only
trigger this mechanism if it already has the ability to disable or
enable an IGP link. The local delay does not increase the negative
consequences. 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.
Litkowski, et al. Standards Track [Page 24]RFC 8333 Micro-loop Prevention by Local Delay March 2018
13. References
13.1. Normative References
[ISO10589] International Organization for Standardization,
"Information technology -- Telecommunications and
information exchange between systems -- Intermediate
System to Intermediate System intra-domain routeing
information exchange protocol for use in conjunction with
the protocol for providing the connectionless-mode network
service (ISO 8473)", ISO/IEC 10589:2002, Second Edition,
November 2002.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998,
<https://www.rfc-editor.org/info/rfc2328>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
13.2. Informative References
[BACKOFF] Decraene, B., Litkowski, S., Gredler, H., Lindem, A.,
Francois, P., and C. Bowers, "SPF Back-off Delay algorithm
for link state IGPs", Work in Progress, draft-ietf-rtgwg-
backoff-algo-10, March 2018.
[PLSN] Zinin, A., "Analysis and Minimization of Microloops in
Link-state Routing Protocols", Work in Progress,
draft-ietf-rtgwg-microloop-analysis-01, October 2005.
[RFC3906] Shen, N. and H. Smit, "Calculating Interior Gateway
Protocol (IGP) Routes Over Traffic Engineering Tunnels",
RFC 3906, DOI 10.17487/RFC3906, October 2004,
<https://www.rfc-editor.org/info/rfc3906>.
[RFC5715] Shand, M. and S. Bryant, "A Framework for Loop-Free
Convergence", RFC 5715, DOI 10.17487/RFC5715, January
2010, <https://www.rfc-editor.org/info/rfc5715>.
Litkowski, et al. Standards Track [Page 25]RFC 8333 Micro-loop Prevention by Local Delay March 2018
[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, <https://www.rfc-editor.org/info/rfc6976>.
[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, <https://www.rfc-editor.org/info/rfc7916>.
Acknowledgements
We would like to thank the authors of [RFC6976] for introducing the
concept of ordered convergence: Mike Shand, Stewart Bryant, Stefano
Previdi, and Olivier Bonaventure.
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
Individual Contributor
Email: pfrpfr@gmail.com
Litkowski, et al. Standards Track [Page 26]
