Network Working Group S. Bryant
Internet-Draft C. Filsfils
Intended status: Standards Track S. Previdi
Expires: November 24, 2013 Cisco Systems
M. Shand
Independent Contributor
N. So
Tata Communications
May 23, 2013
Remote LFA FRR
draft-ietf-rtgwg-remote-lfa-02
Abstract
This draft describes an extension to the basic IP fast re-route
mechanism described in RFC5286 that provides additional backup
connectivity for link failures when none can be provided by the basic
mechanisms.
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 [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 http://datatracker.ietf.org/drafts/current/.
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 November 24, 2013.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://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.
1. Terminology
This draft uses the terms defined in [RFC5714]. This section defines
additional terms used in this draft.
Extended P-space
The union of the P-space of the neighbours of a
specific router with respect to the protected link.
P-space P-space is the set of routers reachable from a
specific router without any path (including equal cost
path splits) transiting the protected link.
For example, the P-space of S, is the set of routers
that S can reach without using the protected link S-E.
PQ node A node which is a member of both the extended P-space
and the Q-space.
Q-space Q-space is the set of routers from which a specific
router can be reached without any path (including
equal cost path splits) transiting the protected link.
Repair tunnel A tunnel established for the purpose of providing a
virtual neighbor which is a Loop Free Alternate.
Remote LFA The tail-end of a repair tunnel. This tail-end is a
member of both the extended-P space the Q space. It
is also termed a "PQ" node.
In this document we use the notation X-Y to mean the path from X to Y
over the link directly connecting X and Y, whilst the notation X->Y
refers to the shortest path from X to Y via some set of unspecified
nodes including the null set (i.e. including over a link directly
connecting X and Y).
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2. Introduction
RFC 5714 [RFC5714] describes a framework for IP Fast Re-route and
provides a summary of various proposed IPFRR solutions. A basic
mechanism using loop-free alternates (LFAs) is described in [RFC5286]
that provides good repair coverage in many
topologies[I-D.filsfils-rtgwg-lfa-applicability], especially those
that are highly meshed. However, some topologies, notably ring based
topologies are not well protected by LFAs alone. This is illustrated
in Figure 1 below.
S---E
/ \
A D
\ /
B---C
Figure 1: A simple ring topology
If all link costs are equal, the link S-E cannot be fully protected
by LFAs. The destination C is an ECMP from S, and so can be
protected when S-E fails, but D and E are not protectable using LFAs
This draft describes extensions to the basic repair mechanism in
which tunnels are used to provide additional logical links which can
then be used as loop free alternates where none exist in the original
topology. For example if a tunnel is provided between S and C as
shown in Figure 2 then C, now being a direct neighbor of S would
become an LFA for D and E. The non-failure traffic distribution is
not disrupted by the provision of such a tunnel since it is only used
for repair traffic and MUST NOT be used for normal traffic.
S---E
/ \ \
A \ D
\ \ /
B---C
Figure 2: The addition of a tunnel
The use of this technique is not restricted to ring based topologies,
but is a general mechanism which can be used to enhance the
protection provided by LFAs.
This technique describes in this document is directed at providing
repairs in the case of link failures. Considerations regarding node
failures are discussed in Section 6.
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3. Repair Paths
As with LFA FRR, when a router detects an adjacent link failure, it
uses one or more repair paths in place of the failed link. Repair
paths are pre-computed in anticipation of later failures so they can
be promptly activated when a failure is detected.
A tunneled repair path tunnels traffic to some staging point in the
network from which it is assumed that, in the absence of multiple
failures, it will travel to its destination using normal forwarding
without looping back. This is equivalent to providing a virtual
loop-free alternate to supplement the physical loop-free alternates.
Hence the name "Remote LFA FRR". When a link cannot be entirely
protected with local LFA neighbors, the protecting router seeks the
help of a remote LFA staging point.
3.1. Tunnels as Repair Paths
Consider an arbitrary protected link S-E. In LFA FRR, if a path to
the destination from a neighbor N of S does not cause a packet to
loop back over the link S-E (i.e. N is a loop-free alternate), then
S can send the packet to N and the packet will be delivered to the
destination using the pre-failure forwarding information. If there
is no such LFA neighbor, then S may be able to create a virtual LFA
by using a tunnel to carry the packet to a point in the network which
is not a direct neighbor of S from which the packet will be delivered
to the destination without looping back to S. In this document such
a tunnel is termed a repair tunnel. The tail-end of this tunnel is
called a "remote LFA" or a "PQ node".
Note that the repair tunnel terminates at some intermediate router
between S and E, and not E itself. This is clearly the case, since
if it were possible to construct a tunnel from S to E then a
conventional LFA would have been sufficient to effect the repair.
3.2. Tunnel Requirements
There are a number of IP in IP tunnel mechanisms that may be used to
fulfil the requirements of this design, such as IP-in-IP [RFC1853]
and GRE[RFC1701] .
In an MPLS enabled network using LDP[RFC5036], a simple label
stack[RFC3032] may be used to provide the required repair tunnel. In
this case the outer label is S's neighbor's label for the repair
tunnel end point, and the inner label is the repair tunnel end
point's label for the packet destination. In order for S to obtain
the correct inner label it is necessary to establish a directed LDP
session[RFC5036] to the tunnel end point.
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The selection of the specific tunnelling mechanism (and any necessary
enhancements) used to provide a repair path is outside the scope of
this document. The authors simply note that deployment in an MPLS/
LDP environment is extremely simple and straight-forward as an LDP
LSP from S to the PQ node is readily available, and hence does not
require any new protocol extension or design change. This LSP is
automatically established as a basic property of LDP behavior. The
performance of the encapsulation and decapsulation is also excellent
as encapsulation is just a push of one label (like conventional MPLS
TE FRR) and the decapsulation occurs naturally at the penultimate hop
before the PQ node.
When a failure is detected, it is necessary to immediately redirect
traffic to the repair path. Consequently, the repair tunnel used
must be provisioned beforehand in anticipation of the failure. Since
the location of the repair tunnels is dynamically determined it is
necessary to establish the repair tunnels without management action.
Multiple repairs may share a tunnel end point.
4. Construction of Repair Paths
4.1. Identifying Required Tunneled Repair Paths
Not all links will require protection using a tunneled repair path.
Referring to Figure 1, if E can already be protected via an LFA, S-E
does not need to be protected using a repair tunnel, since all
destinations normally reachable through E must therefore also be
protectable by an LFA. Such an LFA is frequently termed a "link
LFA". Tunneled repair paths are only required for links which do not
have a link LFA.
4.2. Determining Tunnel End Points
The repair tunnel endpoint needs to be a node in the network
reachable from S without traversing S-E. In addition, the repair
tunnel end point needs to be a node from which packets will normally
flow towards their destination without being attracted back to the
failed link S-E.
Note that once released from the tunnel, the packet will be
forwarded, as normal, on the shortest path from the release point to
its destination. This may result in the packet traversing the router
E at the far end of the protected link S-E., but this is obviously
not required.
The properties that are required of repair tunnel end points are
therefore:
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o The repair tunneled point MUST be reachable from the tunnel source
without traversing the failed link; and
o When released, tunneled packets MUST proceed towards their
destination without being attracted back over the failed link.
Provided both these requirements are met, packets forwarded over the
repair tunnel will reach their destination and will not loop.
In some topologies it will not be possible to find a repair tunnel
endpoint that exhibits both the required properties. For example if
the ring topology illustrated in Figure 1 had a cost of 4 for the
link B-C, while the remaining links were cost 1, then it would not be
possible to establish a tunnel from S to C (without resorting to some
form of source routing).
4.2.1. Computing Repair Paths
The set of routers which can be reached from S without traversing S-E
is termed the P-space of S with respect to the link S-E. The P-space
can be obtained by computing a shortest path tree (SPT) rooted at S
and excising the sub-tree reached via the link S-E (including those
which are members of an ECMP). In the case of Figure 1 the P-space
comprises nodes A and B only. Expressed in cost terms the set of
routers {P} are those for which the shortest path cost S->P is
strictly less than the shortest path cost S->E->P.
The set of routers from which the node E can be reached, by normal
forwarding, without traversing the link S-E is termed the Q-space of
E with respect to the link S-E. The Q-space can be obtained by
computing a reverse shortest path tree (rSPT) rooted at E, with the
sub-tree which traverses the failed link excised (including those
which are members of an ECMP). The rSPT uses the cost towards the
root rather than from it and yields the best paths towards the root
from other nodes in the network. In the case of Figure 1 the Q-space
comprises nodes C and D only. Expressed in cost terms the set of
routers {Q} are those for which the shortest path cost E->Q is
strictly less than the shortest path cost E->S->Q. In Figure 1 the
intersection of the E's Q-space with S's P-space defines the set of
viable repair tunnel end-points, known as "PQ nodes". As can be
seen, for the case of Figure 1 there is no common node and hence no
viable repair tunnel end-point.
Note that the Q-space calculation could be conducted for each
individual destination and a per-destination repair tunnel end point
determined. However this would, in the worst case, require an SPF
computation per destination which is not currently considered to be
scalable. We therefore use the Q-space of E as a proxy for the
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Q-space of each destination. This approximation is obviously correct
since the repair is only used for the set of destinations which were,
prior to the failure, routed through node E. This is analogous to
the use of link-LFAs rather than per-prefix LFAs.
4.2.2. Extended P-space
The description in Section 4.2.1 calculated router S's P-space rooted
at S itself. However, since router S will only use a repair path
when it has detected the failure of the link S-E, the initial hop of
the repair path need not be subject to S's normal forwarding decision
process. Thus we introduce the concept of extended P-space. Router
S's extended P-space is the union of the P-spaces of each of S's
neighbours. This may be calculated by computing the an SPT at each
of S's neighbors (N) (excluding E) and excising the subtree reached
via the path N->S->E. The use of extended P-space may allow router S
to reach potential repair tunnel end points that were otherwise
unreachable. In cost terms a router is in extended P-space if the
shortest path cost S-N->P is strictly less than the shortest path
cost S-E->P.
Another way to describe extended P-space is that it is the union of (
un-extended ) P-space and the set of destinations for which S has a
per-prefix LFA protecting the link S-E. i.e. the repair tunnel end
point can be reached either directly or using a per-prefix LFA.
Since in the case of Figure 1 node A is a per-prefix LFA for the
destination node C, the set of extended P-space nodes comprises nodes
A, B and C. Since node C is also in E's Q-space, there is now a node
common to both extended P-space and Q-space which can be used as a
repair tunnel end-point to protect the link S-E.
4.2.3. Selecting Repair Paths
The mechanisms described above will identify all the possible repair
tunnel end points that can be used to protect a particular link. In
a well-connected network there are likely to be multiple possible
release points for each protected link. All will deliver the packets
correctly so, arguably, it does not matter which is chosen. However,
one repair tunnel end point may be preferred over the others on the
basis of path cost or some other selection criteria.
There is no technical requirement for the selection criteria to be
consistent across all routers, but such consistency may be desirable
from an operational point of view. In general there are advantages
in choosing the repair tunnel end point closest (shortest metric) to
S. Choosing the closest maximises the opportunity for the traffic to
be load balanced once it has been released from the tunnel. For
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consistency in behavior is RECOMMENDED that member of the set of
routers {P} with the lowest cost S->P be the default choice for P.
In the event of a tie the router with the lowest node identifier
SHOULD be selected.
5. Example Application of Remote LFAs
An example of a commonly deployed topology which is not fully
protected by LFAs alone is shown in Figure 3. PE1 and PE2 are
connected in the same site. P1 and P2 may be geographically
separated (inter-site). In order to guarantee the lowest latency
path from/to all other remote PEs, normally the shortest path follows
the geographical distance of the site locations. Therefore, to
ensure this, a lower IGP metric (5) is assigned between PE1 and PE2.
A high metric (1000) is set on the P-PE links to prevent the PEs
being used for transit traffic. The PEs are not individually dual-
homed in order to reduce costs.
This is a common topology in SP networks.
When a failure occurs on the link between PE1 and P2, PE1 does not
have an LFA for traffic reachable via P1. Similarly, by symmetry, if
the link between PE2 and P1 fails, PE2 does not have an LFA for
traffic reachable via P2.
Increasing the metric between PE1 and PE2 to allow the LFA would
impact the normal traffic performance by potentially increasing the
latency.
| 100 |
-P2---------P1-
\ /
1000 \ / 1000
PE1---PE2
5
Figure 3: Example SP topology
Clearly, full protection can be provided, using the techniques
described in this draft, by PE1 choosing P2 as a PQ node, and PE2
choosing P1 as a PQ node.
6. Node Failures
When the failure is a node failure rather than a link failure there
is a danger that the RLFA repair will loop. This is discussed in
detail in [I-D.bryant-ipfrr-tunnels]. In summary problem is that two
of more of E's neighbors each with E as the next hop to some
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destination D may attempt to repair a packet addressed to destination
D via the other neighbor and then E, thus causing a loop to form. As
will be noted from [I-D.bryant-ipfrr-tunnels], this can rapidly
become a complex problem to address.
There are a number of ways to minimize the probability of a loop
forming when a node failure occurs and there exists the possibility
that two of E's neighbors may form a mutual repair.
1. Detect when a packet has arrived on some interface I that is also
the interface used to reach the first hop on the RLFA path to PQ,
and drop the packet. This is useful in the case of a ring
topology.
2. Require that the path from PQ to destination D never passes
through E (including in the ECMP case), i.e. only use node
protecting paths in which the cost PQ to D is strictly less than
the cost PQ to E plus the cost E to D.
3. Require that where the packet may pass through another neighbor
of E, that node is down stream (i.e. strictly closer to D than
the repairing node). This means that some neighbor of E (X) can
repair via some other neighbor of E (Y), but Y cannot repair via
X.
Case 1 accepts that loops may form and suppresses them by dropping
packets. Dropping packets may be considered less detrimental than
looping packets. Cases 2 and 3 above prevent the formation of a
loop, but at the expense of a reduced repair coverage and at the cost
of additional complexity in the algorithm to compute the repair path.
The probability of a node failure and the consequences of node
failure in any particular topology will depend on the node design,
the particular topology in use, and node failure strategy (including
the null strategy). It is recommended that a network operator
perform an analysis of the consequences and probability of node
failure in their network, and determine whether the incidence and
consequence of occurrence are acceptable.
7. Operation in an LDP environment
Where this technique is used in an MPLS network using LDP [RFC5036],
S will need to push two labels onto the repair packet. First it
needs to push PQ's label to the destination, and then it needs to
push its own label for PQ. In the example Section 3.1 S already has
the first hop (B) label for the PQ node (C) as a result of the
ordinary operation of LDP. To get the PQ node (C) label for the
destination (D), S needs to establish a targeted LDP session with C.
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The label stack for normal operation and RLFA operation is shown
below in Figure 4.
+-----------------+ +-----------------+ +-----------------+
| datalink | | datalink | | datalink |
+-----------------+ +-----------------+ +-----------------+
| S's label for D | | E's label for D | | C's label for D |
+-----------------+ +-----------------+ +-----------------+
| Payload | | Payload | | B's label for C |
+-----------------+ +-----------------+ +-----------------+
X Y | Payload |
+-----------------+
Z
X = Normal label stack packet arriving at S
Y = Normal label stack packet leaving S
Z = RLFA label stack to D via C as PQ node
Figure 4
To establish an targeted LDP session with a candidate PQ node the
repairing node (S) needs to know what IP address PQ is willing to use
for targeted LDP sessions. This in turn requires PQ to advertise
this address in the IGP in use. What address is used, how this is
advertised in the IGP, and whether this is a special IP address or an
IP address also used for some other purpose is out of scope for this
document and must be specified in an IGP specific RFC.
8. Historical Note
The basic concepts behind Remote LFA were invented in 2002 and were
later included in [I-D.bryant-ipfrr-tunnels], submitted in 2004.
[I-D.bryant-ipfrr-tunnels], targeted a 100% protection coverage and
hence included additional mechanisms on top of the Remote LFA
concept. The addition of these mechanisms made the proposal very
complex and computationally intensive and it was therefore not
pursued as a working group item.
As explained in [I-D.filsfils-rtgwg-lfa-applicability], the purpose
of the LFA FRR technology is not to provide coverage at any cost. A
solution for this already exists with MPLS TE FRR. MPLS TE FRR is a
mature technology which is able to provide protection in any topology
thanks to the explicit routing capability of MPLS TE.
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The purpose of LFA FRR technology is to provide for a simple FRR
solution when such a solution is possible. The first step along this
simplicity approach was "local" LFA [RFC5286]. We propose "Remote
LFA" as a natural second step. The following section motivates its
benefits in terms of simplicity, incremental deployment and
significant coverage increase.
9. Benefits
Remote LFAs preserve the benefits of RFC5286: simplicity, incremental
deployment and good protection coverage.
9.1. Simplicity
The remote LFA algorithm is simple to compute.
o The extended P space does not require any new computation (it is
known once per-prefix LFA computation is completed).
o The Q-space is a single reverse SPF rooted at the neighbor.
o The directed LDP session is automatically computed and
established.
In edge topologies (square, ring), the directed LDP session position
and number is deterministic and hence troubleshooting is simple.
In core topologies, our simulation indicates that the 90th percentile
number of LDP sessions per node to achieve the significant Remote LFA
coverage observed in section 7.3 is <= 6. This is insignificant
compared to the number of LDP sessions commonly deployed per router
which is frequently is in the several hundreds.
9.2. Incremental Deployment
The establishment of the directed LDP session to the PQ node does not
require any new technology on the PQ node. Indeed, routers commonly
support the ability to accept a remote request to open a directed LDP
session. The new capability is restricted to the Remote-LFA
computing node (the originator of the LDP session).
9.3. Significant Coverage Extension
The previous sections have already explained how Remote LFAs provide
protection for frequently occurring edge topologies: square and
rings. In the core, we extend the analysis framework in section 4.3
of [I-D.filsfils-rtgwg-lfa-applicability]and provide hereafter the
Remote LFA coverage results for the 11 topologies:
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+----------+--------------+----------------+------------+
| Topology | Per-link LFA | Per-prefix LFA | Remote LFA |
+----------+--------------+----------------+------------+
| T1 | 45% | 77% | 78% |
| T2 | 49% | 99% | 100% |
| T3 | 88% | 99% | 99% |
| T4 | 68% | 84% | 92% |
| T5 | 75% | 94% | 99% |
| T6 | 87% | 99% | 100% |
| T7 | 16% | 67% | 96% |
| T8 | 87% | 100% | 100% |
| T9 | 67% | 80% | 98% |
| T10 | 98% | 100% | 100% |
| T11 | 59% | 77% | 95% |
| Average | 67% | 89% | 96% |
| Median | 68% | 94% | 99% |
+----------+--------------+----------------+------------+
Another study[ISOCORE2010] confirms the significant coverage increase
provided by Remote LFAs.
10. Complete Protection
As shown in the previous table, Remote LFA provides for 96% average
(99% median) protection in the 11 analyzed SP topologies.
In an MPLS network, this is achieved without any scalability impact
as the tunnels to the PQ nodes are always present as a property of an
LDP-based deployment.
In the very few cases where P and Q spaces have an empty
intersection, one could select the closest node in the Q space and
signal an explicitely-routed RSVP TE LSP to that Q node. A directed
LDP session is then established with the selected Q node and the rest
of the solution is identical to that described elsewhere in this
document.
The drawbacks of this solution are:
1. only available for MPLS network;
2. the addition of LSPs in the SP infrastructure.
This extension is described for exhaustivity. In practice, the
"Remote LFA" solution should be preferred for three reasons: its
simplicity, its excellent coverage in the analyzed backbones and its
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complete coverage in the most frequent access/aggregation topologies
(box or ring).
11. IANA Considerations
There are no IANA considerations that arise from this architectural
description of IPFRR. The RFC Editor may remove this section on
publication.
12. Security Considerations
The security considerations of RFC 5286 also apply.
To prevent their use as an attack vector the repair tunnel endpoints
SHOULD be assigned from a set of addresses that are not reachable
from outside the routing domain.
13. Acknowledgments
The authors acknowledge the technical contributions made to this work
by Stefano Previdi.
14. Informative References
[I-D.bryant-ipfrr-tunnels]
Bryant, S., Filsfils, C., Previdi, S., and M. Shand, "IP
Fast Reroute using tunnels", draft-bryant-ipfrr-tunnels-03
(work in progress), November 2007.
[I-D.filsfils-rtgwg-lfa-applicability]
Filsfils, C., Francois, P., Shand, M., Decraene, B.,
Uttaro, J., Leymann, N., and M. Horneffer, "LFA
applicability in SP networks", draft-filsfils-rtgwg-lfa-
applicability-00 (work in progress), March 2010.
[ISOCORE2010]
So, N., Lin, T., and C. Chen, "LFA (Loop Free Alternates)
Case Studies in Verizon's LDP Network", 2010.
[RFC1701] Hanks, S., Li, T., Farinacci, D., and P. Traina, "Generic
Routing Encapsulation (GRE)", RFC 1701, October 1994.
[RFC1853] Simpson, W., "IP in IP Tunneling", RFC 1853, October 1995.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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[RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
Encoding", RFC 3032, January 2001.
[RFC5036] Andersson, L., Minei, I., and B. Thomas, "LDP
Specification", RFC 5036, October 2007.
[RFC5286] Atlas, A. and A. Zinin, "Basic Specification for IP Fast
Reroute: Loop-Free Alternates", RFC 5286, September 2008.
[RFC5714] Shand, M. and S. Bryant, "IP Fast Reroute Framework", RFC
5714, January 2010.
Authors' Addresses
Stewart Bryant
Cisco Systems
250, Longwater, Green Park,
Reading RG2 6GB, UK
UK
Email: stbryant@cisco.com
Clarence Filsfils
Cisco Systems
De Kleetlaan 6a
1831 Diegem
Belgium
Email: cfilsfil@cisco.com
Stefano Previdi
Cisco Systems
Email: sprevidi@cisco.com
Mike Shand
Independent Contributor
Email: imc.shand@gmail.com
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Ning So
Tata Communications
Mobile Broadband Services
Email: Ning.So@tatacommunications.com
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