draft-ietf-rtgwg-uloop-delay-06.txt   draft-ietf-rtgwg-uloop-delay-07.txt 
Routing Area Working Group S. Litkowski Routing Area Working Group S. Litkowski
Internet-Draft B. Decraene Internet-Draft B. Decraene
Intended status: Standards Track Orange Intended status: Standards Track Orange
Expires: February 9, 2018 C. Filsfils Expires: April 13, 2018 C. Filsfils
Cisco Systems Cisco Systems
P. Francois P. Francois
Individual Individual
August 8, 2017 October 10, 2017
Micro-loop prevention by introducing a local convergence delay Micro-loop prevention by introducing a local convergence delay
draft-ietf-rtgwg-uloop-delay-06 draft-ietf-rtgwg-uloop-delay-07
Abstract Abstract
This document describes a mechanism for link-state routing protocols This document describes a mechanism for link-state routing protocols
to prevent local transient forwarding loops in case of link failure. to prevent local transient forwarding loops in case of link failure.
This mechanism proposes a two-step convergence by introducing a delay This mechanism proposes a two-step convergence by introducing a delay
between the convergence of the node adjacent to the topology change between the convergence of the node adjacent to the topology change
and the network wide convergence. and the network wide convergence.
As this mechanism delays the IGP convergence it may only be used for As this mechanism delays the IGP convergence it may only be used for
skipping to change at page 1, line 48 skipping to change at page 1, line 48
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on February 9, 2018. This Internet-Draft will expire on April 13, 2018.
Copyright Notice Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Acronyms . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Transient forwarding loops side effects . . . . . . . . . . . 3 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Fast reroute inefficiency . . . . . . . . . . . . . . . . 4 3. Transient forwarding loops side effects . . . . . . . . . . . 4
2.2. Network congestion . . . . . . . . . . . . . . . . . . . 6 3.1. Fast reroute inefficiency . . . . . . . . . . . . . . . . 4
3. Overview of the solution . . . . . . . . . . . . . . . . . . 7 3.2. Network congestion . . . . . . . . . . . . . . . . . . . 7
4. Specification . . . . . . . . . . . . . . . . . . . . . . . . 7 4. Overview of the solution . . . . . . . . . . . . . . . . . . 7
4.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . 7 5. Specification . . . . . . . . . . . . . . . . . . . . . . . . 8
4.2. Current IGP reactions . . . . . . . . . . . . . . . . . . 8 5.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . 8
4.3. Local events . . . . . . . . . . . . . . . . . . . . . . 8 5.2. Current IGP reactions . . . . . . . . . . . . . . . . . . 8
4.4. Local delay for link down . . . . . . . . . . . . . . . . 9 5.3. Local events . . . . . . . . . . . . . . . . . . . . . . 9
5. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 9 5.4. Local delay for link down . . . . . . . . . . . . . . . . 9
5.1. Applicable case: local loops . . . . . . . . . . . . . . 9 6. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 10
5.2. Non applicable case: remote loops . . . . . . . . . . . . 10 6.1. Applicable case: local loops . . . . . . . . . . . . . . 10
6. Simulations . . . . . . . . . . . . . . . . . . . . . . . . . 10 6.2. Non applicable case: remote loops . . . . . . . . . . . . 11
7. Deployment considerations . . . . . . . . . . . . . . . . . . 11 7. Simulations . . . . . . . . . . . . . . . . . . . . . . . . . 11
8. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 12 8. Deployment considerations . . . . . . . . . . . . . . . . . . 12
8.1. Local link down . . . . . . . . . . . . . . . . . . . . . 12 9. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 13
8.2. Local and remote event . . . . . . . . . . . . . . . . . 15 9.1. Local link down . . . . . . . . . . . . . . . . . . . . . 13
8.3. Aborting local delay . . . . . . . . . . . . . . . . . . 17 9.2. Local and remote event . . . . . . . . . . . . . . . . . 17
9. Comparison with other solutions . . . . . . . . . . . . . . . 19 9.3. Aborting local delay . . . . . . . . . . . . . . . . . . 18
9.1. PLSN . . . . . . . . . . . . . . . . . . . . . . . . . . 19 10. Comparison with other solutions . . . . . . . . . . . . . . . 21
9.2. OFIB . . . . . . . . . . . . . . . . . . . . . . . . . . 20 10.1. PLSN . . . . . . . . . . . . . . . . . . . . . . . . . . 21
10. Existing implementations . . . . . . . . . . . . . . . . . . 20 10.2. OFIB . . . . . . . . . . . . . . . . . . . . . . . . . . 21
11. Security Considerations . . . . . . . . . . . . . . . . . . . 21 11. Existing implementations . . . . . . . . . . . . . . . . . . 22
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 21 12. Security Considerations . . . . . . . . . . . . . . . . . . . 22
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21 13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 22
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 21 14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
14.1. Normative References . . . . . . . . . . . . . . . . . . 21 15. References . . . . . . . . . . . . . . . . . . . . . . . . . 22
14.2. Informative References . . . . . . . . . . . . . . . . . 21 15.1. Normative References . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22 15.2. Informative References . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
1. Introduction 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 Micro-forwarding loops and some potential solutions are well
described in [RFC5715]. This document describes a simple targeted described in [RFC5715]. This document describes a simple targeted
mechanism that prevents micro-loops that are local to the failure. mechanism that prevents micro-loops that are local to the failure.
Based on network analysis, local failure make up a significant Based on network analysis, local failures make up a significant
portion of the micro-forwarding loops. A simple and easily portion of the micro-forwarding loops. A simple and easily
deployable solution for these local micro-loops is critical because deployable solution for these local micro-loops is critical because
these local loops cause some traffic loss after a fast-reroute these local loops cause some traffic loss after a fast-reroute
alternate has been used (see Section 2.1). alternate has been used (see Section 3.1).
Consider the case in Figure 1 where S does not have an LFA to protect Consider the case in Figure 1 where S does not have an LFA (Loop Free
its traffic to D. That means that all non-D neighbors of S on the Alternate) to protect its traffic to D. That means that all non-D
topology will send to S any traffic destined to D if a neighbor did neighbors of S on the topology will send to S any traffic destined to
not, then that neighbor would be loop-free. Regardless of the D if a neighbor did not, then that neighbor would be loop-free.
advanced fast-reroute (FRR) technique used, when S converges to the Regardless of the advanced fast-reroute (FRR) technique used, when S
new topology, it will send its traffic to a neighbor that was not converges to the new topology, it will send its traffic to a neighbor
loop-free and thus cause a local micro-loop. The deployment of that was not loop-free and thus cause a local micro-loop. The
advanced fast-reroute techniques motivates this simple router-local deployment of advanced fast-reroute techniques motivates this simple
mechanism to solve this targeted problem. This solution can be work router-local mechanism to solve this targeted problem. This solution
with the various techniques described in [RFC5715]. can be work with the various techniques described in [RFC5715].
1 1
D ------ C D ------ C
| | | |
1 | | 5 1 | | 5
| | | |
S ------ B S ------ B
1 1
Figure 1 Figure 1
When S-D fails, a transient forwarding loop may appear between S and When S-D fails, a transient forwarding loop may appear between S and
B if S updates its forwarding entry to D before B. B if S updates its forwarding entry to D before B.
2. Transient forwarding loops side effects 3. Transient forwarding loops side effects
Even if they are very limited in duration, transient forwarding loops Even if they are very limited in duration, transient forwarding loops
may cause high damages for a network. may cause high damages for a network.
2.1. Fast reroute inefficiency 3.1. Fast reroute inefficiency
D D
1 | 1 |
| 1 | 1
A ------ B A ------ B
| | ^ | | ^
10 | | 5 | T 10 | | 5 | T
| | | | | |
E--------C E--------C
| 1 | 1
1 | 1 |
S S
Figure 2 - RSVP-TE FRR case Figure 2 - RSVP-TE FRR case
In the Figure 2, we consider an IP/LDP routed network. An RSVP-TE 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 tunnel T, provisioned on C and terminating on B, is used to protect
the traffic against C-B link failure (IGP shortcut is activated on the traffic against C-B link failure (the IGP shortcut feature is
C). The primary path of T is C->B and FRR is activated on T activated on C). The primary path of T is C->B and FRR is activated
providing an FRR bypass or detour using path C->E->A->B. On the on T providing an FRR bypass or detour using path C->E->A->B. On
router C, the nexthop to D is the tunnel T thanks to the IGP router C, the next hop to D is the tunnel T thanks to the IGP
shortcut. When C-B link fails: shortcut. When C-B link fails:
1. C detects the failure, and updates the tunnel path using 1. C detects the failure, and updates the tunnel path using a
preprogrammed FRR path, the traffic path from S to D becomes: preprogrammed FRR path. The traffic path from S to D becomes:
S->E->C->E->A->B->A->D. S->E->C->E->A->B->A->D.
2. In parallel, on router C, both the IGP convergence and the TE 2. In parallel, on router C, both the IGP convergence and the TE
tunnel convergence (tunnel path recomputation) are occurring: tunnel convergence (tunnel path recomputation) are occurring:
* The Tunnel T path is recomputed and now uses C->E->A->B. * 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. * The IGP path to D is recomputed and now uses C->E->A->D.
3. On C, the tail-end of the TE tunnel (router B) is no more on the 3. On C, the tail-end of the TE tunnel (router B) is no longer on
shortest-path tree (SPT) to D, so C does not encapsulate anymore the shortest-path tree (SPT) to D, so C does not continue to
the traffic to D using the tunnel T and updates its forwarding encapsulate the traffic to D using the tunnel T and updates its
entry to D using the nexthop E. forwarding entry to D using the nexthop E.
If C updates its forwarding entry to D before router E, there would 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. be a transient forwarding loop between C and E until E has converged.
+-----------+------------+------------------+-----------------------+ +-----------+------------+------------------+-----------------------+
| Network | Time | Router C events | Router E events | | Network | Time | Router C events | Router E events |
| condition | | | | | condition | | | |
+-----------+------------+------------------+-----------------------+ +-----------+------------+------------------+-----------------------+
| S->D | | | | | S->D | | | |
| Traffic | | | | | Traffic | | | |
skipping to change at page 6, line 18 skipping to change at page 6, line 46
| | | | | | | | | |
| | t0+470msec | | E convergence ends | | | t0+470msec | | E convergence ends |
+-----------+------------+------------------+-----------------------+ +-----------+------------+------------------+-----------------------+
Route computation event time scale Route computation event time scale
The issue described here is completely independent of the fast- The issue described here is completely independent of the fast-
reroute mechanism involved (TE FRR, LFA/rLFA, MRT ...) when the reroute mechanism involved (TE FRR, LFA/rLFA, MRT ...) when the
primary path uses hop-by-hop routing. The protection enabled by primary path uses hop-by-hop routing. The protection enabled by
fast-reroute is working perfectly, but ensures a protection, by fast-reroute is working perfectly, but ensures a protection, by
definition, only until the PLR has converged. When implementing FRR, definition, only until the PLR has converged (as soon as the PLR has
a service provider wants to guarantee a very limited loss of converged, it replaces its FRR path by a new primary path). When
connectivity time. The previous example shows that the benefit of implementing FRR, a service provider wants to guarantee a very
FRR may be completely lost due to a transient forwarding loop limited loss of connectivity time. The previous example shows that
appearing when PLR has converged. Delaying FIB updates after the IGP the benefit of FRR may be completely lost due to a transient
convergence may allow to keep the fast-reroute path until the forwarding loop appearing when PLR has converged. Delaying FIB
neighbors have converged and preserves the customer traffic. updates after the IGP convergence may allow to keep the fast-reroute
path until the neighbors have converged and preserves the customer
traffic.
2.2. Network congestion 3.2. Network congestion
1 1
D ------ C D ------ C
| | | |
1 | | 5 1 | | 5
| | | |
A -- S ------ B A -- S ------ B
/ | 1 / | 1
F E F E
Figure 3 Figure 3
In the figure above, as presented in Section 1, when the link S-D 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 fails, a transient forwarding loop may appear between S and B for
destination D. The traffic on the S-B link will constantly increase 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 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 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 link, the S-B link may become congested in a few hundreds of
milliseconds and will stay congested until the loop is eliminated. milliseconds and will stay congested until the loop is eliminated.
The congestion introduced by transient forwarding loops is The congestion introduced by transient forwarding loops is
problematic as it can affect traffic that is not directly affected by problematic as it can affect traffic that is not directly affected by
the failing network component. In our example, the congestion of the the failing network component. In the example, the congestion of the
S-B link will impact some customer traffic that is not directly 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 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 service may mitigate the congestion for some traffic. However, some
traffic not directly affected by the failure will still be dropped as traffic not directly affected by the failure will still be dropped as
a router is not able to distinguish the looping traffic from the a router is not able to distinguish the looping traffic from the
normally forwarded traffic. normally forwarded traffic.
3. Overview of the solution 4. Overview of the solution
This document defines a two-step convergence initiated by the router This document defines a two-step convergence initiated by the router
detecting a failure and advertising the topological changes in the detecting a failure and advertising the topological changes in the
IGP. This introduces a delay between the convergence of the local IGP. This introduces a delay between the convergence of the local
router and the network wide convergence. router and the network wide convergence.
The proposed solution is limited to local link down events in order The proposed solution is limited to local link down events in order
to keep the solution simple. to keep the solution simple.
This ordered convergence, is similar to the ordered FIB proposed This ordered convergence is similar to the ordered FIB proposed
defined in [RFC6976], but it is limited to only a "one hop" distance. defined in [RFC6976], but it is limited to only a "one hop" distance.
As a consequence, it is simpler and becomes a local only feature that As a consequence, it is more simple and becomes a local-only feature
does not require interoperability. This benefit comes at the expense that does not require interoperability. This benefit comes at the
of eliminating transient forwarding loops involving the local router. expense of eliminating transient forwarding loops involving the local
The proposed mechanism also reuses some concepts described in router. The proposed mechanism also reuses some concepts described
[I-D.ietf-rtgwg-microloop-analysis]. in [I-D.ietf-rtgwg-microloop-analysis].
4. Specification 5. Specification
4.1. Definitions 5.1. Definitions
This document will refer to the following existing IGP timers: This document will refer to the following existing IGP timers:
o LSP_GEN_TIMER: The delay used to batch multiple local events in o LSP_GEN_TIMER: The delay used to batch multiple local events in
one single local LSP/LSA update. It is often associated with a one single local LSP/LSA update. It is often associated with a
damping mechanism to slow down reactions by incrementing the timer damping mechanism to slow down reactions by incrementing the timer
when multiple consecutive events are detected. when multiple consecutive events are detected.
o SPF_DELAY: The delay between the first IGP event triggering a new o SPF_DELAY: The delay between the first IGP event triggering a new
routing table computation and the start of that routing table routing table computation and the start of that routing table
computation. It is often associated with a damping mechanism to computation. It is often associated with a damping mechanism to
slow down reactions by incrementing the timer when the IGP becomes slow down reactions by incrementing the timer when the IGP becomes
unstable. As an example, [I-D.ietf-rtgwg-backoff-algo] defines a unstable. As an example, [I-D.ietf-rtgwg-backoff-algo] defines a
standard SPF delay algorithm. standard SPF delay algorithm.
This document introduces the following new timer: This document introduces the following new timer:
o ULOOP_DELAY_DOWN_TIMER: used to slow down the local node o ULOOP_DELAY_DOWN_TIMER: used to slow down the local node
convergence in case of link down events. convergence in case of link down events.
4.2. Current IGP reactions 5.2. Current IGP reactions
Upon a change of the status of an adjacency/link, the existing Upon a change of the status of an adjacency/link, the existing
behavior of the router advertising the event is the following: behavior of the router advertising the event is the following:
1. The Up/Down event is notified to the IGP. 1. The Up/Down event is notified to the IGP.
2. The IGP processes the notification and postpones the reaction for 2. The IGP processes the notification and postpones the reaction for
LSP_GEN_TIMER msec. LSP_GEN_TIMER msec.
3. Upon LSP_GEN_TIMER expiration, the IGP updates its LSP/LSA and 3. Upon LSP_GEN_TIMER expiration, the IGP updates its LSP/LSA and
floods it. floods it.
4. The SPF computation is scheduled in SPF_DELAY msec. 4. The SPF computation is scheduled in SPF_DELAY msec.
5. Upon SPF_DELAY timer expiration, the SPF is computed, then the 5. Upon SPF_DELAY timer expiration, the SPF is computed, then the
RIB and FIB are updated. RIB and FIB are updated.
4.3. Local events 5.3. Local events
The mechanism described in this document assumes that there has been The mechanism described in this document assumes that there has been
a single link failure as seen by the IGP area/level. If this a single link failure as seen by the IGP area/level. If this
assumption is violated (e.g. multiple links or nodes failed), then assumption is violated (e.g. multiple links or nodes failed), then
standard IP convergence MUST be applied (as described in standard IP convergence MUST be applied (as described in
Section 4.2). Section 5.2).
To determine if the mechanism can be applicable or not, an To determine if the mechanism can be applicable or not, an
implementation SHOULD implement logic to correlate the protocol implementation SHOULD implement logic to correlate the protocol
messages (LSP/LSA) received during the SPF scheduling period in order messages (LSP/LSA) received during the SPF scheduling period in order
to determine the topology changes that occured. This is necessary as to determine the topology changes that occured. This is necessary as
multiple protocol messages may describe the same topology change and multiple protocol messages may describe the same topology change and
a single protocol message may describe multiple topology changes. As a single protocol message may describe multiple topology changes. As
a consequence, determining a particular topology change MUST be a consequence, determining a particular topology change MUST be
independent of the order of reception of those protocol messages. independent of the order of reception of those protocol messages.
How the logic works is left to the implementation. How the logic works is left to the implementation.
skipping to change at page 9, line 15 skipping to change at page 9, line 46
Let router B be the computing router when the link B-C fails. B 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 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 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 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 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 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 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 other changes, B can determine that this is a local link failure and
may decide to activate the mechanism described in this document. may decide to activate the mechanism described in this document.
4.4. Local delay for link down 5.4. Local delay for link down
Upon an adjacency/link down event, this document introduces a change Upon an adjacency/link down event, this document introduces a change
in step 5 (Section 4.2) in order to delay the local convergence in step 5 (Section 5.2) in order to delay the local convergence
compared to the network wide convergence. The new step 5 is compared to the network wide convergence. The new step 5 is
described below: described below:
5. Upon SPF_DELAY timer expiration, the SPF is computed. If the 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 condition of a single local link-down event has been met, then an
update of the RIB and the FIB SHOULD be delayed for update of the RIB and the FIB SHOULD be delayed for
ULOOP_DELAY_DOWN_TIMER msecs. Otherwise, the RIB and FIB SHOULD ULOOP_DELAY_DOWN_TIMER msecs. Otherwise, the RIB and FIB SHOULD
be updated immediately. be updated immediately.
If a new convergence occurs while ULOOP_DELAY_DOWN_TIMER is running, 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 ULOOP_DELAY_DOWN_TIMER is stopped and the RIB/FIB SHOULD be updated
as part of the new convergence event. as part of the new convergence event.
As a result of this addition, routers local to the failure will As a result of this addition, routers local to the failure will
converge slower than remote routers. Hence it SHOULD only be done converge slower than remote routers. Hence it SHOULD only be done
for a non-urgent convergence, such as for administrative de- for a non-urgent convergence, such as for administrative de-
activation (maintenance) or when the traffic is protected by fast- activation (maintenance) or when the traffic is protected by fast-
reroute. reroute.
5. Applicability 6. Applicability
As previously stated, this mechanism only avoids the forwarding loops As previously stated, this mechanism only avoids the forwarding loops
on the links between the node local to the failure and its neighbors. on the links between the node local to the failure and its neighbors.
Forwarding loops may still occur on other links. Forwarding loops may still occur on other links.
5.1. Applicable case: local loops 6.1. Applicable case: local loops
A ------ B ----- E A ------ B ----- E
| / | | / |
| / | | / |
G---D------------C F All the links have a metric of 1 G---D------------C F All the links have a metric of 1
Figure 5 Figure 5
Let us consider the traffic from G to F. The primary path is 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 G->D->C->E->F. When link C-E fails, if C updates its forwarding
skipping to change at page 10, line 18 skipping to change at page 11, line 5
as C has FRR enabled and it breaks the FRR forwarding while all as C has FRR enabled and it breaks the FRR forwarding while all
upstream routers are still forwarding the traffic to itself. upstream routers are still forwarding the traffic to itself.
By implementing the mechanism defined in this document on C, when the 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 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 order to allow some time for D to converge. FRR on C keeps
protecting the traffic during this period. When the timer expires on protecting the traffic during this period. When the timer expires on
C, its forwarding entry to F is updated. There is no transient C, its forwarding entry to F is updated. There is no transient
forwarding loop on the link C-D. forwarding loop on the link C-D.
5.2. Non applicable case: remote loops 6.2. Non applicable case: remote loops
A ------ B ----- E --- H A ------ B ----- E --- H
| | | |
| | | |
G---D--------C ------F --- J ---- K G---D--------C ------F --- J ---- K
All the links have a metric of 1 except BE=15 All the links have a metric of 1 except BE=15
Figure 6 Figure 6
skipping to change at page 10, line 47 skipping to change at page 11, line 34
forwarding entry to F is updated. There is no transient forwarding forwarding entry to F is updated. There is no transient forwarding
loop between C and D. However, a transient forwarding loop may still loop between C and D. However, a transient forwarding loop may still
occur between D and A. In this scenario, this mechanism is not occur between D and A. In this scenario, this mechanism is not
enough to address all the possible forwarding loops. However, it enough to address all the possible forwarding loops. However, it
does not create additional traffic loss. Besides, in some cases does not create additional traffic loss. Besides, in some cases
-such as when the nodes update their FIB in the following order C, A, -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- D, for example because the router A is quicker than D to converge-
the mechanism may still avoid the forwarding loop that would have the mechanism may still avoid the forwarding loop that would have
otherwise occurred. otherwise occurred.
6. Simulations 7. Simulations
Simulations have been run on multiple service provider topologies. Simulations have been run on multiple service provider topologies.
+----------+------+ +----------+------+
| Topology | Gain | | Topology | Gain |
+----------+------+ +----------+------+
| T1 | 71% | | T1 | 71% |
| T2 | 81% | | T2 | 81% |
| T3 | 62% | | T3 | 62% |
| T4 | 50% | | T4 | 50% |
skipping to change at page 11, line 46 skipping to change at page 12, line 31
N. N.
o The gain is how many loops (both remote and local) we succeed to o The gain is how many loops (both remote and local) we succeed to
suppress. suppress.
On topology 1, 71% of the transient forwarding loops created by the On topology 1, 71% of the transient forwarding loops created by the
failure of any link are prevented by implementing the local delay. failure of any link are prevented by implementing the local delay.
The analysis shows that all local loops are prevented and only remote The analysis shows that all local loops are prevented and only remote
loops remain. loops remain.
7. Deployment considerations 8. Deployment considerations
Transient forwarding loops have the following drawbacks: Transient forwarding loops have the following drawbacks:
o They limit FRR efficiency: even if FRR is activated within 50msec, 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 as soon as PLR has converged, the traffic may be affected by a
transient loop. transient loop.
o They may impact traffic not directly affected by the failure (due o They may impact traffic not directly affected by the failure (due
to link congestion). to link congestion).
This local delay proposal is a transient forwarding loop avoidance This local delay proposal is a transient forwarding loop avoidance
mechanism (like OFIB). Even if it only addresses local transient mechanism (like OFIB). Even if it only addresses local transient
loops, the efficiency versus complexity comparison of the mechanism loops, the efficiency versus complexity comparison of the mechanism
makes it a good solution. It is also incrementally deployable with makes it a good solution. It is also incrementally deployable with
incremental benefits, which makes it an attractive option both for incremental benefits, which makes it an attractive option both for
vendors to implement and service providers to deploy. Delaying the vendors to implement and service providers to deploy. Delaying the
convergence time is not an issue if we consider that the traffic is convergence time is not an issue if we consider that the traffic is
protected during the convergence. protected during the convergence.
8. Examples 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 : We will consider the following figure for the associated examples :
D D
1 | F----X 1 | F----X
| 1 | | 1 |
A ------ B A ------ B
| | ^ | | ^
10 | | 5 | T 10 | | 5 | T
| | | | | |
skipping to change at page 12, line 39 skipping to change at page 13, line 35
| 1 | 1
1 | 1 |
S S
Figure 7 Figure 7
The network above is considered to have a convergence time about 1 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 second, so ULOOP_DELAY_DOWN_TIMER will be adjusted to this value. We
also consider that FRR is running on each node. also consider that FRR is running on each node.
8.1. Local link down 9.1. Local link down
The table below describes the events and associating timing that The table below describes the events and associated timing that
happens on router C and E when link B-C goes down. As C detects a happens on router C and E when link B-C goes down. As C detects a
single local event corresponding to a link down (its LSP + LSP from B 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 received), it decides to apply the local delay down behavior and no
microloop is formed. microloop is formed.
+-----------+-------------+------------------+----------------------+ +-----------+-------------+------------------+----------------------+
| Network | Time | Router C events | Router E events | | Network | Time | Router C events | Router E events |
| condition | | | | | condition | | | |
+-----------+-------------+------------------+----------------------+ +-----------+-------------+------------------+----------------------+
| S->D | | | | | S->D | | | |
skipping to change at page 15, line 42 skipping to change at page 17, line 5
| | | | | | | | | |
| | t0+1255msec | C updates its | | | | t0+1255msec | C updates its | |
| | | RIB/FIB for D | | | | | RIB/FIB for D | |
| | | | | | | | | |
| | t0+1340msec | C convergence | | | | t0+1340msec | C convergence | |
| | | ends | | | | | ends | |
+-----------+-------------+------------------+----------------------+ +-----------+-------------+------------------+----------------------+
Route computation event time scale Route computation event time scale
8.2. Local and remote event 9.2. Local and remote event
The table below describes the events and associating timing that 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 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 link will fail in the same time window. C will not apply the local
delay because a non local topology change is also received. delay because a non local topology change is also received.
+-----------+------------+-----------------+------------------------+ +-----------+------------+-----------------+------------------------+
| Network | Time | Router C events | Router E events | | Network | Time | Router C events | Router E events |
| condition | | | | | condition | | | |
+-----------+------------+-----------------+------------------------+ +-----------+------------+-----------------+------------------------+
skipping to change at page 17, line 30 skipping to change at page 18, line 40
| | | | | | | | | |
| | t0+450msec | C convergence | | | | t0+450msec | C convergence | |
| | | ends | | | | | ends | |
| | | | | | | | | |
| | t0+470msec | | E convergence ends | | | t0+470msec | | E convergence ends |
| | | | | | | | | |
+-----------+------------+-----------------+------------------------+ +-----------+------------+-----------------+------------------------+
Route computation event time scale Route computation event time scale
8.3. Aborting local delay 9.3. Aborting local delay
The table below describes the events and associated timing that The table below describes the events and associated timing that
happens on router C and E when link B-C goes down. In addition, we 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 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 FIB update. C will first apply the local delay, but when the new
event happens, it will fall back to the standard convergence event happens, it will fall back to the standard convergence
mechanism without delaying route insertion anymore. In this example, mechanism without further delaying route insertion. In this example,
we consider a ULOOP_DELAY_DOWN_TIMER configured to 2 seconds. we consider a ULOOP_DELAY_DOWN_TIMER configured to 2 seconds.
+-----------+------------+-------------------+----------------------+ +-----------+------------+-------------------+----------------------+
| Network | Time | Router C events | Router E events | | Network | Time | Router C events | Router E events |
| condition | | | | | condition | | | |
+-----------+------------+-------------------+----------------------+ +-----------+------------+-------------------+----------------------+
| S->D | | | | | S->D | | | |
| Traffic | | | | | Traffic | | | |
| OK | | | | | OK | | | |
| | | | | | | | | |
skipping to change at page 19, line 37 skipping to change at page 21, line 5
| OK | | | | | OK | | | |
| | | | | | | | | |
| | t0+781msec | C convergence | | | | t0+781msec | C convergence | |
| | | ends | | | | | ends | |
| | | | | | | | | |
| | t0+810msec | | E convergence ends | | | t0+810msec | | E convergence ends |
+-----------+------------+-------------------+----------------------+ +-----------+------------+-------------------+----------------------+
Route computation event time scale Route computation event time scale
9. Comparison with other solutions 10. Comparison with other solutions
As stated in Section 3, our solution reuses some concepts already As stated in Section 4, the proposed solution reuses some concepts
introduced by other IETF proposals but tries to find a tradeoff already introduced by other IETF proposals but tries to find a
between efficiency and simplicity. This section tries to compare tradeoff between efficiency and simplicity. This section tries to
behaviors of the solutions. compare behaviors of the solutions.
9.1. PLSN 10.1. PLSN
PLSN ([I-D.ietf-rtgwg-microloop-analysis]) describes a mechanism PLSN ([I-D.ietf-rtgwg-microloop-analysis]) describes a mechanism
where each node in the network tries to avoid transient forwarding where each node in the network tries to avoid transient forwarding
loops upon a topology change by always keeping traffic on a loop-free 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 path for a defined duration (locked path to a safe neighbor). The
locked path may be the new primary nexthop, another neighbor, or the locked path may be the new primary nexthop, another neighbor, or the
old primary nexthop depending how the safety condition is satisfied. old primary nexthop depending how the safety condition is satisfied.
PLSN does not solve all transient forwarding loops (see PLSN does not solve all transient forwarding loops (see
[I-D.ietf-rtgwg-microloop-analysis] Section 4 for more details). [I-D.ietf-rtgwg-microloop-analysis] Section 4 for more details).
Our solution reuses some concept of PLSN but in a more simple Our solution reuses some concept of PLSN but in a more simple
fashion: fashion:
o PLSN has three different behaviors: keep using old nexthop, use o PLSN has three different behaviors: keep using old nexthop, use
new primary nexthop if it is safe, or use another safe nexthop, new primary nexthop if it is safe, or use another safe nexthop,
while our solution only have one: keep using the current nexthop while the proposed solution only has one: keep using the current
(old primary, or already activated FRR path). nexthop (old primary, or already activated FRR path).
o PLSN may cause some damage while using a safe nexthop which is not 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 the new primary nexthop in case the new safe nexthop does not
enough provide enough bandwidth (see [RFC7916]). Our solution may provide enough bandwidth (see [RFC7916]). This solution may not
not experience this issue as the service provider may have control experience this issue as the service provider may have control on
on the FRR path being used preventing network congestion. the FRR path being used preventing network congestion.
o PLSN applies to all nodes in a network (remote or local changes), o PLSN applies to all nodes in a network (remote or local changes),
while our mechanism applies only on the nodes connected to the while the proposed mechanism applies only on the nodes connected
topology change. to the topology change.
9.2. OFIB 10.2. OFIB
OFIB ([RFC6976]) describes a mechanism where the convergence of the OFIB ([RFC6976]) describes a mechanism where the convergence of the
network upon a topology change is made ordered to prevent transient network upon a topology change is ordered in order to prevent
forwarding loops. Each router in the network must deduce the failure transient forwarding loops. Each router in the network must deduce
type from the LSA/LSP received and computes/applies a specific FIB the failure type from the LSA/LSP received and computes/applies a
update timer based on the failure type and its rank in the network specific FIB update timer based on the failure type and its rank in
considering the failure point as root. the network considering the failure point as root.
This mechanism allows to solve all the transient forwarding loop in a This mechanism allows to solve all the transient forwarding loop in a
network at the price of introducing complexity in the convergence network at the price of introducing complexity in the convergence
process that may require a strong monitoring by the service provider. process that may require a strong monitoring by the service provider.
Our solution reuses the OFIB concept but limits it to the first hop Our solution reuses the OFIB concept but limits it to the first hop
that experiences the topology change. As demonstrated, our proposal that experiences the topology change. As demonstrated, the mechanism
allows to solve all the local transient forwarding loops that proposed in this document allows to solve all the local transient
represents an high percentage of all the loops. Moreover limiting forwarding loops that represents an high percentage of all the loops.
the mechanism to one hop allows to keep the network-wide convergence Moreover limiting the mechanism to one hop allows to keep the
behavior. network-wide convergence behavior.
10. Existing implementations 11. Existing implementations
At this time, there are three different implementations of this At this time, there are three different implementations of this
mechanism: CISCO IOS-XR, CISCO IOS-XE and Juniper JUNOS. The three mechanism: CISCO IOS-XR, CISCO IOS-XE and Juniper JUNOS. The three
implementations have been tested in labs and demonstrated a good implementations have been tested in labs and demonstrated good
behavior in term of local micro-loop avoidance. The feature has also behavior in term of local micro-loop avoidance. The feature has also
been deployed in some live networks. No side effects have been been deployed in some live networks. No side effects have been
found. found.
11. Security Considerations 12. Security Considerations
This document does not introduce any change in term of IGP security. This document does not introduce any change in term of IGP security.
The operation is internal to the router. The local delay does not The operation is internal to the router. The local delay does not
increase the number of attack vectors as an attacker could only increase the number of attack vectors as an attacker could only
trigger this mechanism if he already has be ability to disable or trigger this mechanism if he already has be ability to disable or
enable an IGP link. The local delay does not increase the negative enable an IGP link. The local delay does not increase the negative
consequences. If an attacker has the ability to disable or enable an consequences. If an attacker has the ability to disable or enable an
IGP link, it can already harm the network by creating instability and IGP link, it can already harm the network by creating instability and
harm the traffic by creating forwarding packet loss and forwarding harm the traffic by creating forwarding packet loss and forwarding
loss for the traffic crossing that link. loss for the traffic crossing that link.
12. Acknowledgements 13. Acknowledgements
We would like to thanks the authors of [RFC6976] for introducing the We would like to thanks the authors of [RFC6976] for introducing the
concept of ordered convergence: Mike Shand, Stewart Bryant, Stefano concept of ordered convergence: Mike Shand, Stewart Bryant, Stefano
Previdi, and Olivier Bonaventure. Previdi, and Olivier Bonaventure.
13. IANA Considerations 14. IANA Considerations
This document has no actions for IANA. This document has no actions for IANA.
14. References 15. References
15.1. Normative References
14.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
14.2. Informative References 15.2. Informative References
[I-D.ietf-rtgwg-backoff-algo] [I-D.ietf-rtgwg-backoff-algo]
Decraene, B., Litkowski, S., Gredler, H., Lindem, A., Decraene, B., Litkowski, S., Gredler, H., Lindem, A.,
Francois, P., and C. Bowers, "SPF Back-off algorithm for Francois, P., and C. Bowers, "SPF Back-off algorithm for
link state IGPs", draft-ietf-rtgwg-backoff-algo-05 (work link state IGPs", draft-ietf-rtgwg-backoff-algo-05 (work
in progress), May 2017. in progress), May 2017.
[I-D.ietf-rtgwg-microloop-analysis] [I-D.ietf-rtgwg-microloop-analysis]
Zinin, A., "Analysis and Minimization of Microloops in Zinin, A., "Analysis and Minimization of Microloops in
Link-state Routing Protocols", draft-ietf-rtgwg-microloop- Link-state Routing Protocols", draft-ietf-rtgwg-microloop-
analysis-01 (work in progress), October 2005. analysis-01 (work in progress), October 2005.
[RFC5715] Shand, M. and S. Bryant, "A Framework for Loop-Free [RFC5715] Shand, M. and S. Bryant, "A Framework for Loop-Free
Convergence", RFC 5715, DOI 10.17487/RFC5715, January Convergence", RFC 5715, DOI 10.17487/RFC5715, January
2010, <http://www.rfc-editor.org/info/rfc5715>. 2010, <https://www.rfc-editor.org/info/rfc5715>.
[RFC6976] Shand, M., Bryant, S., Previdi, S., Filsfils, C., [RFC6976] Shand, M., Bryant, S., Previdi, S., Filsfils, C.,
Francois, P., and O. Bonaventure, "Framework for Loop-Free Francois, P., and O. Bonaventure, "Framework for Loop-Free
Convergence Using the Ordered Forwarding Information Base Convergence Using the Ordered Forwarding Information Base
(oFIB) Approach", RFC 6976, DOI 10.17487/RFC6976, July (oFIB) Approach", RFC 6976, DOI 10.17487/RFC6976, July
2013, <http://www.rfc-editor.org/info/rfc6976>. 2013, <https://www.rfc-editor.org/info/rfc6976>.
[RFC7916] Litkowski, S., Ed., Decraene, B., Filsfils, C., Raza, K., [RFC7916] Litkowski, S., Ed., Decraene, B., Filsfils, C., Raza, K.,
Horneffer, M., and P. Sarkar, "Operational Management of Horneffer, M., and P. Sarkar, "Operational Management of
Loop-Free Alternates", RFC 7916, DOI 10.17487/RFC7916, Loop-Free Alternates", RFC 7916, DOI 10.17487/RFC7916,
July 2016, <http://www.rfc-editor.org/info/rfc7916>. July 2016, <https://www.rfc-editor.org/info/rfc7916>.
Authors' Addresses Authors' Addresses
Stephane Litkowski Stephane Litkowski
Orange Orange
Email: stephane.litkowski@orange.com Email: stephane.litkowski@orange.com
Bruno Decraene Bruno Decraene
Orange Orange
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