| Routing Area Working Group | S. Litkowski |
| Internet-Draft | B. Decraene |
| Intended status: Standards Track | Orange |
| Expires: July 26, 2014 | C. Filsfils |
| K. Raza | |
| Cisco Systems | |
| M. Horneffer | |
| Deutsche Telekom | |
| P. Sarkar | |
| Juniper Networks | |
| January 22, 2014 |
Operational management of Loop Free Alternates
draft-ietf-rtgwg-lfa-manageability-01
Loop Free Alternates (LFA), as defined in RFC 5286 is an IP Fast ReRoute (IP FRR) mechanism enabling traffic protection for IP traffic (and MPLS LDP traffic by extension). Following first deployment experiences, this document provides operational feedback on LFA, highlights some limitations, and proposes a set of refinements to address those limitations. It also proposes required management specifications.
This proposal is also applicable to remote LFA solution.
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].
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This Internet-Draft will expire on July 26, 2014.
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Following the first deployments of Loop Free Alternates (LFA), this document provides feedback to the community about the management of LFA.
[RFC5286] introduces the notion of tie breakers when selecting the LFA among multiple candidate alternate next-hops. When multiple LFA exist, RFC 5286 has favored the selection of the LFA providing the best coverage of the failure cases. While this is indeed a goal, this is one among multiple and in some deployment this lead to the selection of a suboptimal LFA. The following sections details real use cases of such limitations.
Note that the use case of per-prefix LFA is assumed throughout this analysis.
R1 --------- R2 ---------- R3 --------- R4 | 1 100 1 | | | | 100 | 100 | | | 1 100 1 | R5 --------- R6 ---------- R7 --------- R8 -- R9 - PE1 | | | | | 5k | 5k | 5k | 5k | | | | +--- n*PEx ---+ +---- PE2 ----+ | | PEy Figure 1
Rx routers are core routers using n*10G links. PEs are connected using links with lower bandwidth.
In figure 1, let us consider the traffic flowing from PE1 to PEx. The nominal path is R9-R8-R7-R6-PEx. Let us consider the failure of link R7-R8. For R8, R4 is not an LFA and the only available LFA is PE2.
When the core link R8-R7 fails, R8 switches all traffic destined to all the PEx towards the edge node PE2. Hence an edge node and edge links are used to protect the failure of a core link. Typically, edge links have less capacity than core links and congestion may occur on PE2 links. Note that although PE2 was not directly affected by the failure, its links become congested and its traffic will suffer from the congestion.
In summary, in case of failure, the impact on customer traffic is:
Besides the congestion aspects of using an Edge router as an alternate to protect a core failure, a service provider may consider this as a bad routing design and would like to prevent it.
R1 --------- R2 ------------ R3 --------- R4 | 1 100 | 1 | | | | | 100 | 30 | 30 | | | | 1 50 50 | 10 | R5 -------- R6 ---- R10 ---- R7 -------- R8 --- R9 - PE1 | | \ | | 5000 | 5000 \ 5000 | 5000 | | \ | +--- n*PEx --+ +----- PE2 ----+ | | PEy Figure 2
Rx routers are core routers meshed with n*10G links. PEs are meshed using links with lower bandwidth.
In the figure 2, let us consider the traffic coming from PE1 to PEx. Nominal path is R9-R8-R7-R6-PEx. Let us consider the failure of the link R7-R8. For R8, R4 is a link-protecting LFA and PE2 is a node-protecting LFA. PE2 is chosen as best LFA due to its better protection type. Just like in case 1, this may lead to congestion on PE2 links upon LFA activation.
+--- PE3 --+
/ \
1000 / \ 1000
/ \
+----- R1 ---------------- R2 ----+
| | 500 | |
| 10 | | | 10
| | | |
R5 | 10 | 10 R7
| | | |
| 10 | | | 10
| | 500 | |
+---- R3 ---------------- R4 -----+
\ /
\ /
\ /
+--- PE1 ---+
Figure 3
Rx routers are core routers. R1-R2 and R3-R4 links are 1G links. All others inter Rx links are 10G links.
In the figure above, let us consider the failure of link R1-R3. For destination PE3, R3 has two possible alternates:
R4 is chosen as best LFA due to its better protection type. However, it may not be desirable to use R4 for bandwidth capacity reason. A service provider may prefer to use high bandwidth links as prefered LFA. In this example, prefering shortest path over protection type may achieve the expected behavior, but in cases where metric are not reflecting bandwidth, it would not work and some other criteria would need to be involved when selecting the best LFA.
P1 P2
| \ / |
50 | 50 \/ 50 | 50
| /\ |
PE1-+ +-- PE2
\ /
45 \ / 45
-PE3-+
(OL set)
Figure 4
In the figure above, PE3 has its overload bit set (permanently, for design reason) and wants to protect traffic using LFA for destination PE2.
On PE3, the loopfree condition is not satisified : 100 !< 45 + 45. PE1 is thus not considered as an LFA. However thanks to the overload bit set on PE3, we know that PE1 is loopfree so PE1 is an LFA to reach PE2.
In case of overload condition set on a node, LFA behavior must be clarified.
Controlling best alternate and LFA activation granularity is a requirement for Service Providers. This section defines configuration requirements for LFA.
The granularity of LFA activation should be controlled (as alternate nexthop consume memory in forwarding plane).
An implementation of LFA SHOULD allow its activation with the following criteria:
When multiple alternates exist, LFA selection algorithm is based on tie breakers. Current tie breakers do not provide sufficient control on how the best alternate is chosen. This document proposes an enhanced tie breaker allowing service providers to manage all specific cases:
In addition to direct LFAs, tunnels (e.g. IP, LDP or RSVP-TE) to distant routers may be used to complement LFA coverage (tunnel tail used as virtual neighbor). When a router has multiple alternate candidates for a specific destination, it may have connected alternates and remote alternates reachable via a tunnel. Connected alternates may not always provide an optimal routing path and it may be preferable to select a remote alternate over a connected alternate. The usage of tunnels to extend LFA coverage is described in [I-D.ietf-rtgwg-remote-lfa].
In figure 1, there is no core alternate for R8 to reach PEs located behind R6, so R8 is using PE2 as alternate, which may generate congestion when FRR is activated. Instead, we could have a remote core alternate for R8 to protect PEs destinations. For example, a tunnel from R8 to R3 would ensure LFA protection without using an edge router to protect a core router.
When selecting the best alternate, the selection algorithm MUST consider all available alternates (connected or tunnel). Especially, computation of PQ set ([I-D.ietf-rtgwg-remote-lfa]) SHOULD to be performed before best alternate selection.
An implementation of LFA MUST support the following criteria:
An implementation of LFA SHOULD support the following enhanced criteria:
The policy to select the best alternate evaluate multiple criterions (e.g. metric, SRLG, link colors ...) which first need to be computed for each alternate.. In order to compare the different alternate path, a router must retrieve the attributes of each alternate path. The alternate path is composed of two distinct parts : PLR to alternate and alternate to destination.
For alternate path using a connected alternate :
For alternate path using a remote alternate (tunnel) :
The number of remote alternates may be very high, simulations shown that hundred's of PQs may exist for a single interface being protected. Running a forward SPF for every PQ-node in the network is not scalable.
To handle this situation, it is needed to limit the number of remote alternates to be evaluated to a finite number before collecting alternate path attributes and running the policy evaluation. [I-D.psarkar-rtgwg-rlfa-node-protection] Section 2.3.3 provides a way to reduce the number of PQ to be evaluated.
Link Remote Remote
alternate alternate alternate
------------- ------------------ -------------
Alternates | LFA | | rLFA (PQs) | | Static |
sources | | | | | tunnels |
------------- ------------------ -------------
| | |
| | |
| ---------------------- |
| | Prune some PQs | |
| | (sorting strategy) | |
| ---------------------- |
| | |
| | |
------------------------------------------------
| Collect alternate attributes |
------------------------------------------------
|
|
-------------------------
| Evaluate policy |
-------------------------
|
|
Best alternates
10
PE2 - PE3
| |
50 | 5 | 50
P1----P2
\\ //
50 \\ // 50
PE1
Figure 5
Links between P1 and PE1 are L1 and L2, links between P2 and PE1 are L3 and L4
In the figure above, primary path from PE1 to PE2 is through P1 using ECMP on two parallel links L1 and L2. In case of standard ECMP behavior, if L1 is failing, postconvergence nexthop would become L2 and there would be no longer ECMP. If LFA is activated, as stated in [RFC5286] Section 3.4., "alternate next-hops may themselves also be primary next-hops, but need not be" and "alternate next-hops should maximize the coverage of the failure cases". In this scenario there is no alternate providing node protection, LFA will so prefer L2 as alternate to protect L1 which makes sense compared to postconvergence behavior.
Considering a different scenario using figure 5, where L1 and L2 are configured as a layer 3 bundle using a local feature, as well as L3/L4 being a second layer 3 bundle. Layer 3 bundles are configured as if a link in the bundle is failing, the traffic must be rerouted out of the bundle. Layer 3 bundles are generally introduced to increase bandwidth between nodes. In nominal situation, ECMP is still available from PE1 to PE2, but if L1 is failing, postconvergence nexthop would become ECMP on L3 and L4. In this case, LFA behavior SHOULD be adapted in order to reflect the bandwidth requirement.
We would expect the following FIB entry on PE1 :
On PE1 : PE2 +--> ECMP -> L1
| |
| +----> L2
|
+--> LFA(ECMP) -> L3
|
+---------> L4
If L1 or L2 is failing, traffic must be switched on the LFA ECMP bundle rather than using the other primary nexthop.
As mentioned in [RFC5286] Section 3.4., protecting a link within an ECMP by another primary nexthop is not a MUST. Moreover, we already presented in this document, that maximizing the coverage of the failure case may not be the right approach and policy based choice of alternate may be preferred.
An implementation SHOULD permit to prefer a primary nexthop by another primary nexthop with the possibility to deactivate this criteria. An implementation SHOULD permit to use an ECMP bundle as a LFA.
[RFC5286] Section 3. proposes to reuse GMPLS IGP extensions to encode SRLGs ([RFC4205] and [RFC4203]). The section is also describing the algorithm to compute SRLG protection.
When SRLG protection is computed, and implementation SHOULD permit to :
When applying SRLG criteria, the SRLG violation check SHOULD be performed on source to alternate as well as alternate to destination paths. In the case of remote LFA, PQ to destination path attributes would be retrieved from SPT rooted at PQ.
Link coloring is a powerful system to control the choice of alternates. Protecting interfaces are tagged with colors. Protected interfaces are configured to include some colors with a preference level, and exclude others.
Link color information SHOULD be signalled in the IGP. How signalling is done is out of scope of the document but it may be useful to reuse existing admin-groups from traffic-engineering extensions.
PE2
| +---- P4
| /
PE1 ---- P1 --------- P2
| 10Gb
1Gb |
|
P3
Figure 5
P1 is configured to protect the P1-P4 link. We assume that given the topology, all neighbors are candidate LFA. We would like to enforce a policy in the network where only a core router may protect against the failure of a core link, and where high capacity links are prefered.
In this example, we can use the proposed link coloring by:
Using this, PE links will never be used to protect against P1-P4 link failure and 10Gb link will be be preferred.
The main advantage of this solution is that it can easily be duplicated on other interfaces and other nodes without change. A Service Provider has only to define the color system (associate color with a significance), as it is done already for TE affinities or BGP communities.
An implementation of link coloring:
As mentionned in previous sections, not taking into account bandwidth of an alternate could lead to congestion during FRR activation. We propose to base the bandwidth criteria on the link speed information for the following reason :
Based on this, it is not useful to gather available bandwidth on alternate paths, as the router does not know how much bandwidth it requires for protection. The proposed link speed approach provides a good approximation with a small cost as information is easily available.
The bandwidth criteria of the policy framework SHOULD work in two ways :
Rather than tagging interface on each node (using link color) to identify neighbor node type (as example), it would be helpful if routers could be identified in the IGP. This would permit a grouped processing on multiple nodes. [I-D.psarkar-isis-node-admin-tag] proposes a tagging mechanism for ISIS nodes that may help the node identification. As an implementation need to exclude some specific neighbors (see Section 3.2.3), an implementation :
A specific neighbor may be identified by its interface or IP address and group of neighbors may be identified by a marker like SUB-TLV1 in TLV135. As multiple prefixes may be present in TLVs 135, an heuristic is required to choose the appropriate one that will identify the neighbor and will transport the tag associated with the neighbor preference.
We propose the following algorithm to select the prefix :
PE3
|
|
PE2
| +---- P4
| /
PE1 ---- P1 -------- P2
| 10Gb
1Gb |
|
P3
Figure 6
Consider the following network:
A simple policy could be configured on P1 to choose the best alternate for P1->P4 based on router function/role as follows :
In [RFC5286], Section 3.5, the setting of the overload bit condition in LFA computation is only taken into account for the case where a neighbor has the overload bit set.
In addition to RFC 5286 inequality 1 Loop-Free Criterion (Distance_opt(N, D) < Distance_opt(N, S) + Distance_opt(S, D)), the IS-IS overload bit of the LFA calculating neighbor (S) SHOULD be taken into account. Indeed, if it has the overload bit set, no neighbor will loop back to traffic to itself.
Service providers often perform manual link shutdown (using router CLI) to perform some network changes/tests. Especially testing or troubleshooting FRR requires to perform the manual shutdown on the remote end of the link as generally a local shutdown would not trigger FRR. To enhance such situation, an implementation SHOULD support triggering/activating LFA Fast Reroute for a given link when a manual shutdown is done.
LFA introduction requires some enhancement in standard routing information provided by implementations. Moreover, due to the non 100% coverage, coverage informations is also required.
Hence an implementation :
It is pretty easy to evaluate the coverage of a network in a nominal situation, but topology changes may change the coverage. In some situations, the network may no longer be able to provide the required level of protection. Hence, it becomes very important for service providers to get alerted about changes of coverage.
An implementation SHOULD :
An implementation MAY :
Although the procedures for providing alerts are beyond the scope of this document, we recommend that implementations consider standard and well used mechanisms like syslog or SNMP traps.
The operator may choose to run simulations in order to ensure full coverage of a certain type for the whole network or a given subset of the network. This is particularly likely if he operates the network in the sense of the third backbone profiles described in [RFC6571], that is, he seeks to design and engineer the network topology in a way that a certain coverage is always achieved. Obviously a complete and exact simulation of the IP FRR coverage can only be achieved, if the behavior is deterministic and if the algorithm used is available to the simulation tool. Thus, an implementation SHOULD:
This document does not introduce any change in security consideration compared to [RFC5286].
Significant contributions were made by Pierre Francois, Hannes Gredler, Chris Bowers, Jeff Tantsura and Mustapha Aissaoui which the authors would like to acknowledge.
This document has no action for IANA.
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. |
| [RFC5286] | Atlas, A. and A. Zinin, "Basic Specification for IP Fast Reroute: Loop-Free Alternates", RFC 5286, September 2008. |
| [RFC4205] | Kompella, K. and Y. Rekhter, "Intermediate System to Intermediate System (IS-IS) Extensions in Support of Generalized Multi-Protocol Label Switching (GMPLS)", RFC 4205, October 2005. |
| [RFC4203] | Kompella, K. and Y. Rekhter, "OSPF Extensions in Support of Generalized Multi-Protocol Label Switching (GMPLS)", RFC 4203, October 2005. |