Internet DRAFT - draft-ietf-rtgwg-uloop-delay
draft-ietf-rtgwg-uloop-delay
Routing Area Working Group S. Litkowski
Internet-Draft B. Decraene
Intended status: Standards Track Orange
Expires: May 16, 2018 C. Filsfils
Cisco Systems
P. Francois
Individual
November 12, 2017
Micro-loop prevention by introducing a local convergence delay
draft-ietf-rtgwg-uloop-delay-09
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-step 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 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.
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 https://datatracker.ietf.org/drafts/current/.
Litkowski, et al. Expires May 16, 2018 [Page 1]
Internet-Draft uloop-delay November 2017
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 May 16, 2018.
Copyright Notice
Copyright (c) 2017 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
(https://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. Acronyms . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Transient forwarding loops side effects . . . . . . . . . . . 4
3.1. Fast reroute inefficiency . . . . . . . . . . . . . . . . 4
3.2. Network congestion . . . . . . . . . . . . . . . . . . . 7
4. Overview of the solution . . . . . . . . . . . . . . . . . . 7
5. Specification . . . . . . . . . . . . . . . . . . . . . . . . 8
5.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . 8
5.2. Regular IGP reaction . . . . . . . . . . . . . . . . . . 8
5.3. Local events . . . . . . . . . . . . . . . . . . . . . . 9
5.4. Local delay for link down . . . . . . . . . . . . . . . . 10
6. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 10
6.1. Applicable case: local loops . . . . . . . . . . . . . . 10
6.2. Non applicable case: remote loops . . . . . . . . . . . . 11
7. Simulations . . . . . . . . . . . . . . . . . . . . . . . . . 11
8. Deployment considerations . . . . . . . . . . . . . . . . . . 12
9. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 13
9.1. Local link down . . . . . . . . . . . . . . . . . . . . . 14
9.2. Local and remote event . . . . . . . . . . . . . . . . . 18
9.3. Aborting local delay . . . . . . . . . . . . . . . . . . 19
10. Comparison with other solutions . . . . . . . . . . . . . . . 23
10.1. PLSN . . . . . . . . . . . . . . . . . . . . . . . . . . 23
10.2. OFIB . . . . . . . . . . . . . . . . . . . . . . . . . . 23
11. Implementation Status . . . . . . . . . . . . . . . . . . . . 24
Litkowski, et al. Expires May 16, 2018 [Page 2]
Internet-Draft uloop-delay November 2017
12. Security Considerations . . . . . . . . . . . . . . . . . . . 25
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 25
14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 26
15.1. Normative References . . . . . . . . . . . . . . . . . . 26
15.2. Informative References . . . . . . . . . . . . . . . . . 26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 27
1. Acronyms
FIB: Forwarding Information Base
FRR: Fast ReRoute
IGP: Interior Gateway Protocol
LFA: Loop Free Alternate
LSA: Link State Advertisement
LSP: Link State Packet
MRT: Maximum Redundant Trees
OFIB: Ordered FIB
PLSN: Path Locking via Safe Neighbor
RIB: Routing Information Base
RLFA: Remote Loop Free Alternate
SPF: Shortest Path First
TTL: Time To Live
2. Introduction
Micro-forwarding loops and some potential solutions are well
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 failures make up 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 3.1).
Litkowski, et al. Expires May 16, 2018 [Page 3]
Internet-Draft uloop-delay November 2017
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 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 work with the various techniques
described in [RFC5715].
D ------ C
| |
| | 5
| |
S ------ B
Figure 1
In the Figure 1, all links have a metric of 1 except B-C which has a
metric of 5. When S-D fails, a transient forwarding loop may appear
between S and B if S updates its forwarding entry to D before B does.
3. Transient forwarding loops side effects
Even if they are very limited in duration, transient forwarding loops
may cause significant network damage.
3.1. 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, we consider an IP/LDP routed network. An RSVP-TE
tunnel T, provisioned on C and terminating on B, is used to protect
Litkowski, et al. Expires May 16, 2018 [Page 4]
Internet-Draft uloop-delay November 2017
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 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 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.
The table 1 below 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+20msec | C detects the | |
| | | failure | |
| | | | |
| S->D | t0+40msec | C activates FRR | |
| Traffic | | | |
| OK | | | |
| | | | |
Litkowski, et al. Expires May 16, 2018 [Page 5]
Internet-Draft uloop-delay November 2017
| | 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 |
| Traffic | | | for D |
| OK | | | |
| | | | |
| | t0+470msec | | E convergence ends |
+-----------+------------+------------------+-----------------------+
Table 1 - Route computation event time scale
The issue described here is completely independent of the fast-
reroute mechanism involved (TE FRR, LFA/rLFA, MRT ...) when the
primary path uses hop-by-hop routing. The protection enabled by
fast-reroute is working perfectly, but ensures a protection, by
Litkowski, et al. Expires May 16, 2018 [Page 6]
Internet-Draft uloop-delay November 2017
definition, only until the PLR has converged (as soon as the PLR has
converged, it replaces its FRR path by a new primary path). 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.
3.2. Network congestion
1
D ------ C
| |
1 | | 5
| |
A -- S ------ B
/ | 1
F E
Figure 3
In the figure above, as presented in Section 2, 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 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.
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 the example, the congestion of the
S-B link will impact some customer traffic that is not directly
affected by the failure: e.g. A to B, F to B, 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.
4. Overview of the solution
This document defines a two-step convergence initiated by the router
detecting a failure and advertising the topological changes in the
IGP. This introduces a delay between network-wide convergence and
the convergence of the local router.
Litkowski, et al. Expires May 16, 2018 [Page 7]
Internet-Draft uloop-delay November 2017
The proposed solution is limited to local link down events in order
to keep the solution simple.
This ordered convergence is similar to the ordered FIB proposed
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 proposed mechanism also reuses some concepts
described in [I-D.ietf-rtgwg-microloop-analysis].
5. Specification
5.1. Definitions
This document will refer 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 two consecutives local LSP/LSA
generation. From an operational point of view, this delay is
usually tuned to batch multiple local events in one single local
LSP/LSA update. In IS-IS, this timer is defined as
minimumLSPGenerationInterval in [ISO10589]. In OSPF version 2,
this timer is defined as MinLSInterval in [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, [I-D.ietf-rtgwg-backoff-algo] defines a
standard SPF (Shortest Path First) 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.
5.2. Regular IGP reaction
Upon a change of the status of an adjacency/link, 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.
Litkowski, et al. Expires May 16, 2018 [Page 8]
Internet-Draft uloop-delay November 2017
2. The IGP processes the notification and postpones the reaction for
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 timer expiration, the SPF is computed, 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 can be 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 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 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.
Example:
+--- E ----+--------+
| | |
A ---- B -------- C ------ D
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 LSP/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
Litkowski, et al. Expires May 16, 2018 [Page 9]
Internet-Draft uloop-delay November 2017
other changes, B can determine that this is a local link failure and
may decide to activate the mechanism described in this document.
5.4. Local delay for link down
Upon an adjacency/link down event, this document introduces a change
in step 5 (Section 5.2) in order to delay the local convergence
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 msecs. 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 for administrative de-
activation (maintenance) or when the traffic is protected by fast-
reroute.
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.
6.1. Applicable case: local loops
A ------ B ----- E
| / |
| / |
G---D------------C F All the links have a metric of 1
Figure 5
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.
Litkowski, et al. Expires May 16, 2018 [Page 10]
Internet-Draft uloop-delay November 2017
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 the 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
A ------ B ----- E --- H
| |
| |
G---D--------C ------F --- J ---- K
All the links have a metric of 1 except BE=15
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.
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 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 would have
otherwise occurred.
7. Simulations
Simulations have been run on multiple service provider topologies.
Litkowski, et al. Expires May 16, 2018 [Page 11]
Internet-Draft uloop-delay November 2017
+----------+------+
| Topology | Gain |
+----------+------+
| T1 | 71% |
| T2 | 81% |
| T3 | 62% |
| T4 | 50% |
| T5 | 70% |
| T6 | 70% |
| T7 | 59% |
| T8 | 77% |
+----------+------+
Table 2 - 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 neighbors 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 many loops (both 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 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 50msec,
as soon as PLR has converged, the traffic may be affected by a
transient loop.
Litkowski, et al. Expires May 16, 2018 [Page 12]
Internet-Draft uloop-delay November 2017
o They may impact traffic not directly affected 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 both for
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).
The proposed mechanism is limited to link down events. When a link
goes down, it eventually goes back up. As a consequence, with the
proposed 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 the transient
forwarding loops during the link up event, it should take care of
using specific procedures to bring the link back online. As
examples, the operator can decide to put back the link online out of
business hours or it can use some incremental metric changes to
prevent loops (as proposed in [RFC5715]).
9. Examples
We will consider the following figure for the associated examples :
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 about 1
second, so ULOOP_DELAY_DOWN_TIMER will be adjusted to this value. We
also consider that FRR is running on each node.
Litkowski, et al. Expires May 16, 2018 [Page 13]
Internet-Draft uloop-delay November 2017
9.1. Local link down
The table 3 describes the events and associated timing that happen on
router C and E when link B-C goes down. It is based on a theoretical
sequence of event that should only been read as an example. As C
detects a single local event corresponding to a link down (its LSP +
LSP from B received), it applies the local delay down behavior and no
microloop is formed.
Litkowski, et al. Expires May 16, 2018 [Page 14]
Internet-Draft uloop-delay November 2017
+-----------+-------------+------------------+----------------------+
| 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 | |
| | | 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 |
Litkowski, et al. Expires May 16, 2018 [Page 15]
Internet-Draft uloop-delay November 2017
| | | | 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 | |
+-----------+-------------+------------------+----------------------+
Table 3 - 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. The 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+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 | |
Litkowski, et al. Expires May 16, 2018 [Page 16]
Internet-Draft uloop-delay November 2017
| 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 |
| | | | 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 | |
+-----------+-------------+------------------+----------------------+
Table 4 - Route computation event time scale
Litkowski, et al. Expires May 16, 2018 [Page 17]
Internet-Draft uloop-delay November 2017
9.2. Local and remote event
The table 5 describes the events and associated timing that happen 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. The table 5 is
based on a theoretical sequence of event 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+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 | |
Litkowski, et al. Expires May 16, 2018 [Page 18]
Internet-Draft uloop-delay November 2017
| | | 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 | |
| | | ends | |
| | | | |
| | t0+470msec | | E convergence ends |
| | | | |
+-----------+------------+-----------------+------------------------+
Table 5 - Route computation event time scale
9.3. Aborting local delay
The table 6 describes the events and associated timing that happen on
router C and E when link B-C goes down. In addition, we consider
what happens when 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. The table
Litkowski, et al. Expires May 16, 2018 [Page 19]
Internet-Draft uloop-delay November 2017
6 is based on a theoretical sequence of event that should only been
read as an example.
Litkowski, et al. Expires May 16, 2018 [Page 20]
Internet-Draft uloop-delay November 2017
+-----------+------------+-------------------+----------------------+
| 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 | |
| | | 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 |
Litkowski, et al. Expires May 16, 2018 [Page 21]
Internet-Draft uloop-delay November 2017
| | | | 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 | |
| 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 |
+-----------+------------+-------------------+----------------------+
Table 6 - Route computation event time scale
Litkowski, et al. Expires May 16, 2018 [Page 22]
Internet-Draft uloop-delay November 2017
10. Comparison with other solutions
As stated in Section 4, the proposed 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.
10.1. 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.
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 the proposed solution only has 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
provide enough bandwidth (see [RFC7916]). This 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 the proposed mechanism applies only on 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 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.
Litkowski, et al. Expires May 16, 2018 [Page 23]
Internet-Draft uloop-delay November 2017
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, the mechanism
proposed in this document 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.
11. Implementation Status
At this time, there are three different implementations of this
mechanism.
o Implementation 1:
* Organization: Cisco
* Implementation name: Local Microloop Protection
* Operating system: IOS-XE
* Level of maturity: production release
* Coverage: all the specification is implemented
* Protocols supported: ISIS and OSPF
* Implementation experience: tested in lab and works as expected
* Comment: the feature gives the ability to choose to apply the
delay to FRR protected entry only
* Report last update: 10-11-2017
o Implementation 2:
* Organization: Cisco
* Implementation name: Local Microloop Protection
* Operating system: IOS-XR
* Level of maturity: deployed
* Coverage: all the specification is implemented
Litkowski, et al. Expires May 16, 2018 [Page 24]
Internet-Draft uloop-delay November 2017
* Protocols supported: ISIS and OSPF
* Implementation experience: deployed and works as expected
* Comment: the feature gives the ability to choose to apply the
delay to FRR protected entry only
* Report last update: 10-11-2017
o Implementation 3:
* Organization: Juniper Networks
* Implementation name: Microloop avoidance when IS-IS link fails
* Operating system: JUNOS
* Level of maturity: deployed (hidden command)
* Coverage: all the specification is implemented
* Protocols supported: ISIS only
* Implementation experience: deployed and works as expected
* Comment: the feature applies to all the ISIS routes
* Report last update: 10-11-2017
12. 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 number of attack vectors 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. 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.
13. 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.
Litkowski, et al. Expires May 16, 2018 [Page 25]
Internet-Draft uloop-delay November 2017
14. IANA Considerations
This document has no actions for IANA.
15. References
15.1. Normative References
[ISO10589]
"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 10589, 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>.
15.2. 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-06 (work
in progress), October 2017.
[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.
[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. Expires May 16, 2018 [Page 26]
Internet-Draft uloop-delay November 2017
[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>.
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
Email: pfrpfr@gmail.com
Litkowski, et al. Expires May 16, 2018 [Page 27]
