| Routing Area Working Group | S. Litkowski |
| Internet-Draft | B. Decraene |
| Intended status: Standards Track | Orange |
| Expires: May 17, 2013 | C. Fils Fils |
| K. Raza | |
| Cisco Systems | |
| November 13, 2012 |
Interactions between LFA and RSVP-TE
draft-litkowski-rtgwg-lfa-rsvpte-cooperation-00
RSVP-TE FRR is a well known and deployed technology for fast reroute, and there are some use cases where LFA and RSVP-TE may interact. This document clarifies the behavior of LFA when RSVP-TE tunnels are used simultaneously.
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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].
This document is being discussed on the rtgwg@ietf.org mailing list.
When a failure occurs inside an IP network, network has to converge. This convergence leads to traffic disruption. Some mechanisms are already available to limit traffic disruption by computing alternate path and switching locally to alternate path as soon as failure is detected. All this can be termed as a protection mechanism. Currently, the widely used protection mechanisms for traffic are (i) RSVP-TE Fast Reroute [RFC4090] and Loop Free Alternates [RFC5286]. Both have pros and cons. For example, RSVP-TE FRR permits full network coverage but with a quite high complexity in terms of operation as well as potential scaling issues. Whereas, LFA is a very easy, manageable and quite scalable mechanism but it does not provides full coverage.
This document presents some scenarios where LFA and RSVP-TE may interact. The document also proposes the usage of RSVP-TE tunnels to extend LFA coverage and clarifies interactions between the two protection mechanisms.
This section provides the summarized normative requirements for the interaction between LFA FRR and TE FRR. Following sections provide illustration and reasoning why an operator may care for these behavior.
If a node N has an IGP/LDP forwarding entry F1 with outgoing interface i1 and an IGP/LDP forwarding entry F2 with outgoing interface onto a TE tunnel T2 (due to IGP shortcut [RFC3906]) and tunnel T2 has outgoing interface i2, then an implementation MUST support enabling LFA FRR for F1 and using TE FRR for F2 as long as i1 != i2.
If i1 == i2, an implementation SHOULD allow for using LFA FRR backup for F1 and TE FRR backup for F2.
TE tunnel LFA candidate mechanism is the ability to use a TE tunnel as a virtual neighbor in LFA computation.
If a node N has an IGP/LDP forwarding entry F1 with outgoing interface i1 and N originates a TE tunnel T1 terminating at node Y, then an implementation SHOULD support a local policy which instructs node N to consider Y as a virtual neighbor and hence include Y as part of the LFA FRR alternate computation. In such case, an implementation MUST not use Y as an LFA for F1 if T1's outgoing interface is i1.
This would permit to extend LFA coverage as described in [I-D.ietf-rtgwg-remote-lfa]
The mechanisms for using TE tunnel as an LFA candidate, and RFC3906 mechanisms MUST be de-correlated -- i.e an implementation MUST support TE tunnel configuration with RFC3906 only, as LFA candidate only, or both at the same time.
If a node N has an IGP/LDP forwarding entry F1 with outgoing interface i1, and N originates a TE tunnel T2 terminating on direct neighbor N2 (for example : if a TE tunnel is provisionned for link protection), T2 has an outgoing interface i2 and N2 is best LFA for F1, then an implementation MUST NOT use T2 when programming LFA repair for F1 unless T2 is configured as an LFA candidate.
If a node N has an IGP/LDP forwarding entry F1 with outgoing interface i1 and a MPLS TE midpoint forwarding entry F2 with outgoing interface i2, then an implementation MUST support using LFA FRR for F1 and TE FRR for F2 as long as i1 != i2.
If i1 == i2, an implementation SHOULD allow for using LFA FRR backup for F1 and TE FRR backup for F2.
This section describes some of the deployment scenarios where LFA and RSVP-TE may interact.
There are many networks where RSVP-TE is already deployed. The deployment of RSVP-TE is typically for two main reasons :
LFA is a feature that may bring benefits on RSVP-TE enabled networks, with no/minimal operational cost (compared to RSVP-TE FRR global roll out). These benefits include:
For IP networks that do not have any traffic protection mechanism, LFA is a very good first step to provide traffic protection even if its coverage is not 100%.
Providers may want to increase protection coverage if LFA benefit is not sufficient for some destinations. The following sections describe usage of basic RSVP-TE tunnels to extend protection coverage.
R4 --(10)- R5 -(1)- R6 -(10)- R7
| |
(10) ---(30)----R14--(10)----- (10)
| / \ |
R1 -R2 -(100)-- R3 --(22)--- R8 ---(10)------- R9 --(100)-- R10 ---- R11
/ / | |
(10) (10) (40) |
| / | |
R16-(200)-R15 R12 --(1)-- R13--------(5)------
Figure 1
In figure 1, shortest path from R1 to R11 is : R1-R2-R3-R8-R9-R10-R11.
An RSVP-TE tunnel (tunnel1) is configured on R3 towards R10 using constrained path R4-R5-R6-R7-R9-R10, and another tunnel (tunnel2) is configured towards R8 using IGP path (with no constraints). IGP shortcut ([RFC3906]) is activated on R3, leading to the following forwarding informations (not exhaustive) on R3 :
Per destination LFA is also activated on R3
Based on the normative principles already described, here are the FRR backup paths :
R4 --(10)-- R5 -(1)- R6 -(10)- R7
| | |
(10) (60) (10)
| | |
R1 -R2 -(100)-- R3 --(22)--- R8 ---(10)------- R9 --(100)-- R10 ---- R11
| |
(40) |
| |
R12 --(1)-- R13--------(5)------
Figure 3
In figure 3, RSVP-TE tunnel is configured from R3 to R9 using constrained path R4-R5-R6-R7. R4 to R7 routers are midpoints of tunnels, and they are receiving tunneled packets as well as native IP packets on their interface. We consider LFA implemented on R5.
As explained in normative principles, IP prefixes will be programmed with LFA alternate as backup entry (if available) and MPLS ILM (Incoming label map) of transit TE tunnel will not use LFA alternate as backup NHLFE even if there is a LFA to reach tunnel tail-end (TE use his own protection).
In our figure 3, R5 receives native IP traffic from R4 to R10 and tunneled traffic from R1 to R11 (tunneling done by R3). R5 computes LFAs for all destinations. R8 is an alternate to reach R10, as well as to reach R11 and R9. When link R5->R6 fails, traffic from R4 is switched to alternate R8 (protection by LFA), but traffic from R1 to R11 may be dropped (in case of tunnel without protection) or switched to RSVP-TE protection path (in case tunnel has protection).
We already have seen in previous section that creating RSVP-TE tunnels would increase LFA coverage. The choice of tunnel placement should depend on what type of protection (link or node) we want to achieve, as well as on the set of destinations we want to protect.
Some topologies like rings do not work well with LFA. We consider the following topology in our document for further illustration.
(30 routers) --- R1 ----(30)---- R2 --- (50 routers)
| |
(5) (30)
| |
R3 R4 --- (20 routers)
| |
(10) (30)
| |
------- R5 ------
Figure 5
In this topology, we consider LFA enabled on all routers and links. We will focus on R5 view of the network.
R5 is routing all traffic towards R3 as nominal nexthop, except traffic towards R4 which is routed using R4 (direct link) as nexthop. R5 has LFAs only for R2 and destinations behind R2 (primary nexthop R3, alternate R4). There are 104 destinations (routers) in the network in figure 5. R5-R3 link has 49% LFA coverage with 83 destinations primarily routed on it. R3 node (from R5 point of view) has 49% LFA coverage with 83 destinations primarily routed through it from R5. R5-R4 link has 0% coverage with 21 destinations primarily routed on it. R4 node (from R5 point of view) has 0% coverage with 21 destinations primarily routed on it through it from R5.
Globally from R5 point of view, LFA is able to protect to partially (49%) protect from failure of R3, while failure of R4 is not protected at all.
From R5 point of view, traffic routed through R3 is protected (node protected) at about 49% with 83 destinations routed on it. Destinations R1,R3 and routers behind R1 are not protected, while R2 and destinations behind it are protected.
To extend the coverage, the idea is to extend the topology using LFA tunnel candidate mechanism. This mechanism is of a local significance only.
The hardest task is then to choose the tunnel(s) endpoint such as to achieve the maximum coverage. Tunnel definition must satisfy :
The approach to choose tunnel endpoints might be different here when compared to [I-D.ietf-rtgwg-remote-lfa] as endpoint choice is a manual one. Automatic behavior and scaling of [I-D.ietf-rtgwg-remote-lfa] requires : [I-D.ietf-rtgwg-remote-lfa] may : [I-D.ietf-rtgwg-remote-lfa] and provides more flexibility:
Based on this,
The proposed solution of manual explicitly routed tunnels is a good complement for
From manageability aspect of our manual solution, computing a best Q node for each destination could lead to have one different Q node for different destination. This is not optimal in terms of number of tunnels, given that possibly one Q node may be able to serve multiple non covered destinations.
Rather than computing the best Q node per non covered destination, we would prefer to find best compromise Q nodes (best for multiple destinations). To find the best compromise between coverage increase and number of tunnels, we recommend to use a simulator that do the following computation per link :
Multiple endpoint (and so tunnels) could be necessary to have 100% coverage. But the idea is to find a tradeoff between number of tunnels configured (complexity) and number of destination covered, combining with traffic information would also provide a better view. Globally, configuring 2 tunnels providing 80% coverage would be considered as (operationnally) better than configuring 15 tunnels to obtain 100% coverage.
In our example, we can create a tunnel from R5 to R2. R2 satisfies node protection equation for destination R1, routers behind R1 and R3. To satisfy the second criteria, the tunnel must not use IGP path but must be constrained through R4. With this, we achieve 100% of LFA coverage for R3 router failure.
TBD.
This document has no actions 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. |
| [RFC3906] | Shen, N. and H. Smit, "Calculating Interior Gateway Protocol (IGP) Routes Over Traffic Engineering Tunnels", RFC 3906, October 2004. |
| [RFC4090] | Pan, P., Swallow, G. and A. Atlas, "Fast Reroute Extensions to RSVP-TE for LSP Tunnels", RFC 4090, May 2005. |
| [I-D.ietf-rtgwg-remote-lfa] | Bryant, S, Filsfils, C, Shand, M and S Ning, "Remote LFA FRR", Internet-Draft draft-ietf-rtgwg-remote-lfa-00, June 2012. |
| [I-D.bryant-ipfrr-tunnels] | Bryant, S, Filsfils, C, Previdi, S and M Shand, "IP Fast Reroute using tunnels", Internet-Draft draft-bryant-ipfrr-tunnels-03, November 2007. |