Network Working Group S. Bryant
Internet-Draft C. Filsfils
Intended status: Standards Track S. Previdi
Expires: May 26, 2014 Cisco Systems
M. Shand
Independent Contributor
N. So
Tata Communications
November 22, 2013
Remote LFA FRR
draft-ietf-rtgwg-remote-lfa-04
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 May 26, 2014.
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
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Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Repair Paths . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Tunnels as Repair Paths . . . . . . . . . . . . . . . . . 5
3.2. Tunnel Requirements . . . . . . . . . . . . . . . . . . . 5
4. Construction of Repair Paths . . . . . . . . . . . . . . . . 6
4.1. Identifying Required Tunneled Repair Paths . . . . . . . 6
4.2. Determining Tunnel End Points . . . . . . . . . . . . . . 6
4.2.1. Computing Repair Paths . . . . . . . . . . . . . . . 7
4.2.2. Selecting Repair Paths . . . . . . . . . . . . . . . 9
4.3. A Cost Based RLFA Algorithm . . . . . . . . . . . . . . . 10
5. Example Application of Remote LFAs . . . . . . . . . . . . . 14
6. Node Failures . . . . . . . . . . . . . . . . . . . . . . . . 15
7. Operation in an LDP environment . . . . . . . . . . . . . . . 16
8. Analysis of Real World Topologies . . . . . . . . . . . . . . 17
8.1. Topology Details . . . . . . . . . . . . . . . . . . . . 17
8.2. LFA only . . . . . . . . . . . . . . . . . . . . . . . . 18
8.3. RLFA . . . . . . . . . . . . . . . . . . . . . . . . . . 18
8.4. Comparison of LFA an RLFA results . . . . . . . . . . . . 20
9. Management Considerations . . . . . . . . . . . . . . . . . . 21
10. Historical Note . . . . . . . . . . . . . . . . . . . . . . . 21
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
12. Security Considerations . . . . . . . . . . . . . . . . . . . 21
13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 22
14. Informative References . . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24
1. Terminology
This draft uses the terms defined in [RFC5714]. This section defines
additional terms used in this draft.
Extended P-space
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The union of the P-space of the neighbours of a
specific router with respect to the protected link
(see Section 4.2.1.2).
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. In remote LFA this is used
as the repair tunnel endpoint.
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 (RLFA) The use of a PQ node rather than a neighbour of
the repairing node as the next hop in an LFA repair.
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).
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[RFC6571],
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.
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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. A study of the protection achieved
using remote LFA in typical service provider core networks is
provided in Section 8, and a side by side comparison between LFA and
remote LFA is provided in Section 8.4.
Remote LFA is suitable for incremental deployment within a network,
including a network that is already deploying LFA. Computation of
the repair path is relatively simple, and takes place exclusively on
the repairing node. In MPLS networks the targeted LDP protocol
needed to learn the label binding at the repair tunnel endpoint is a
well understood and widely deployed technology.
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". In its simplest form, when a link
cannot be entirely protected with local LFA neighbors, the protecting
router seeks the help of a remote LFA staging point. Network
manageability considerations may lead to a repair strategy that uses
a remote LFA more frequently [I-D.ietf-rtgwg-lfa-manageability].]
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 (the
repair tunnel endpoint) is a "PQ node" and the repair mechanism is a
"remote LFA".
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
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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.
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 deployment in an MPLS/LDP environment is
relatively simple in the data plane as an LDP LSP from S to the
repair tunnel endpoint (the selected 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 efficient 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 repair tunnel endpoint. In the
control plane, a targeted LDP (TLDP) session is needed between the
repairing node and the repair tunnel endpoint, which will need to be
established and the labels processed before the tunnel can be used.
The time to establish the TLDP session and acquire labels will limit
the speed at which a new tunnel can be put into service, but this
will not be a problem in normal operation.
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 automatically establish the repair tunnels. 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.
It should be noted that using the Q-space of E as a proxy for the
Q-space of each destination can result in failing to identify valid
remote LFAs. The extent to which this reduces the effective
protection coverage is topology dependent.
4.2. Determining Tunnel End Points
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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:
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
To compute the repair path for link S-E we need to determine the set
of routers which can be reached from S without traversing S-E, and
match this with the set of routers from which the node E can be
reached, by normal forwarding, without traversing the link S-E.
We will proceed as follows: we will describe how to compute the set
of routers which can be reached from S without traversing S-E. We
call this the S's P-space with respect to the failure of link S-E. We
will then note that S is able to use P-Space of its neighbours since
S can determine which neighbour it will use as the next hop for the
repair. We call this the S's Extended P-Space with respect to the
failure of link S-E. The use of extended P-Space allows greater
repair coverage and is the preferred approach. Finally we will show
how to compute the set of routers from which the node E can be
reached, by normal forwarding, without traversing the link S-E. This
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is called the Q-space of E with respect to the link S-E. The
selection of the preferred node from the set of nodes that an in both
Extended P-Space and Q-Space is described in Section 4.2.2.
A suitable cost based algorithm to compute the set of nodes common to
both extended P-space and Q-space is provide in Section 4.3.
4.2.1.1. P-space
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.
4.2.1.2. Extended P-space
The description in Section 4.2.1.1 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 (N). This may be calculated by computing an SPT at
each of S's neighbors (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 (P) is in extended P-space if
the shortest path cost N->P is strictly less than the shortest path
cost N->S->E->P. In other words, once the packet it forced to N by S,
it is lower cost for it to continue on to P by any path except one
that takes it back to S and then across the S->E link.
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.
4.2.1.3. Q-space
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
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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 Q->E is
strictly less than the shortest path cost Q->S->E. 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
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. 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
consistency in behavior, is RECOMMENDED that member of the set of
routers {PQ} 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.
It is a local matter whether the repair path selection policy used by
the router favours LFA repairs over RLFA repairs. An LFA repair has
the advantage of not requiring the use of tunnel, however network
manageability considerations may lead to a repair strategy that uses
a remote LFA more frequently [I-D.ietf-rtgwg-lfa-manageability].
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As described in [RFC5286], always selecting a PQ node that is
downstream with respect to the repairing node, prevents the formation
of loops when the failure is worse than expected. The use of
downstream nodes reduces the repair coverage, and operators are
advised to determine whether adequate coverage is achieved before
enabling this selection feature.
4.3. A Cost Based RLFA Algorithm
The preceding text has mostly described the computation of the remote
LFA repair target (PQ) in terms of the intersection of two
reachability graphs computed using SPFs. This section describes a
method of computing the remote LFA repair target using a cost based
algorithm. The pseudo-code provides in this section avoids
unnecessary SPF computations, but for the sake of readability, it
does not otherwise try to optimize the code. In this description
D_opt(a,b) is the shortest distance from node a to node b as computed
by the SPF.
The following notation is used:
o D_opt(a,b) is the shortest distance from node a to node b as
computed by the SPF.
o dest is the packet destination
o fail_intf is the failed interface (S-E in the example)
o fail_intf.remote_node is the node reachable over interface
fail_intf (node E in the example)
o intf.remote_node is the node reachable over interface intf
o root is the root of the SPF calculation
o self is the node carrying out the computation
o y is the node in the network under consideration
//////////////////////////////////////////////////////////////////
//
// Main Function
//////////////////////////////////////////////////////////////////
//
// We have already computed the forward SPF from self to all nodes
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// y in network and thus we know D_opt (self, y). This is needed
// for normal forwarding.
// However for completeness.
Compute_and_Store_Forward_SPF(self)
// To extend P-space we compute the SPF at each neighbour except
// the neighbour that is reached via the link being protected.
// We will also need D_opt(fail_intf.remote_node,y) so compute
// that at the same time.
Compute_Neighbor_SPFs()
// Compute the set of nodes {P} reachable other than via the
// failed link
Compute_Extended_P_Space(fail_intf)
// Compute the set of nodes that can reach the node on the far
// side of the failed link without traversing the failed link.
Compute_Q_Space(fail_intf)
// Compute the set of candidate RLFA tunnel endpoints
Intersect_Extended_P_and_Q_Space()
// Make sure that we cannot get looping repairs when the
// failure is worse than expected.
if (guarantee_no_looping_on_worse_than_protected_failure)
Apply_Downstream_Constraint()
//
// End of Main Function
//
//////////////////////////////////////////////////////////////////
//////////////////////////////////////////////////////////////////
//
// Procedures
//
/////////////////////////////////////////////////////////////////
//
// This computes the SPF from root, and stores the optimum
// distance from root to each node y
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Compute_and_Store_Forward_SPF(root)
Compute_Forward_SPF(root)
foreach node y in network
store D_opt(root,y)
/////////////////////////////////////////////////////////////////
//
// This computes the optimum distance from each neighbour (other
// than the neighbour reachable through the failed link) and
// every other node in the network
Compute_Neighbor_SPFs()
foreach interface intf in self
Compute_and_Store_Forward_SPF(intf.remote_node)
/////////////////////////////////////////////////////////////////
//
// The reverse SPF computes the cost from each remote node to
// root. This is achieved by running the normal SPF algorithm,
// but using the link cost in the direction from the next hop
// back towards root in place of the link cost in the direction
// away from root towards the next hop.
Compute_and_Store_Reverse_SPF(root)
Compute_Reverse_SPF(root)
foreach node y in network
store D_opt(y,root)
/////////////////////////////////////////////////////////////////
//
// Calculate extended P-space
//
// Note the strictly less than operator is needed to
// avoid ECMP issues.
Compute_Extended_P_Space(fail_intf)
foreach node y in network
y.in_extended_P_space = false
// Extend P-space to the P-spaces of all reachable
// neighbours
foreach interface intf in self
if (intf.remote_node != fail_intf.remote_node)
if ( D_opt(intf.remote_node, y) <
D_opt(intf.remote_node, self) +
D_opt(self,fail_intf.remote_node) +
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D_opt(fail_intf.remote_node,y) )
y.in_extended_P_space = true
/////////////////////////////////////////////////////////////////
//
// Compute the nodes in Q-space
//
Compute_Q_Space(fail_intf)
// Compute the cost from every node the network to the
// node normally reachable across the failed link
Compute_and_Store_Reverse_SPF(fail_intf.remote_node)
// Compute the cost from every node the network to self
Compute_and_Store_Reverse_SPF(self)
foreach node y in network
if ( D_opt(y,fail_intf.remote_node) < D_opt(y,self) +
D_opt(self,fail_intf.remote_node) )
y.in_Q_space = true
else
y.in_Q_space = false
/////////////////////////////////////////////////////////////////
//
// Compute set of nodes in both extended P-space and in Q-space
Intersect_Extended_P_and_Q_Space()
foreach node y in network
if ( y.in_extended_P_space && y.in_Q_space )
y.valid_tunnel_endpoint = true
else
y.valid_tunnel_endpoint = false
/////////////////////////////////////////////////////////////////
//
// A downstream route is one where the next hop is strictly
// closer to the destination. By sending the packet to a
// PQ node that is downstream, we know that if the PQ node
// detects a failure, it will not loop the packet back to self.
// This is useful when there are two failures, or a node has
// failed rather than a link.
Apply_Downstream_Constraint()
foreach node y in network
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if (y.valid_tunnel_endpoint)
Compute_and_Store_Forward_SPF(y)
if ((D_opt(y,dest) < D_opt(self,dest))
y.valid_tunnel_endpoint = true
else
y.valid_tunnel_endpoint = false
//
/////////////////////////////////////////////////////////////////
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
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Clearly, full protection can be provided, using the techniques
described in this draft, by PE1 choosing P1 as the remote LFA repair
target node, and PE2 choosing P2 as the remote LFA repair target.
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
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 the
remote LFA repair target, and drop the packet. This is useful in
the case of a ring topology.
2. Require that the path from the remote LFA repair target to
destination D never passes through E (including in the ECMP
case), i.e. only use node protecting paths in which the cost from
the remote LFA repair target to D is strictly less than the cost
from the remote LFA repair target 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. This approach may also lead to dropping some
legitimate 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
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failure in their network, and determine whether the incidence and
consequence of occurrence are acceptable.
This topic is further discussed in
[I-D.psarkar-rtgwg-rlfa-node-protection].
7. Operation in an LDP environment
Where this technique is used in an MPLS network using LDP [RFC5036],
and S is a transit node, S will need to swap the top label in the
stack for the emote LFA repair target's (PQ's) label to the
destination, and to then push its own label for the remote LFA repair
target.
In the example Section 2 S already has the first hop (A) label for
the remote LFA repair target (C) as a result of the ordinary
operation of LDP. To get the remote LFA repair target's label (C's
label) for the destination (D), S needs to establish a targeted LDP
session with C. 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 | | A's label for C |
+-----------------+ +-----------------+ +-----------------+
| Payload | | Payload | | C's label for D |
+-----------------+ +-----------------+ +-----------------+
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 the remote LFA repair target.
Figure 4
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To establish an targeted LDP session with a candidate remote LFA
repair target node the repairing node (S) needs to know what IP
address that the remote LFA repair target is willing to use for
targeted LDP sessions. Ideally this is provided by the remote LFA
repair target advertising this address in the IGP in use. Which
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.
In the absence of a protocol to learn the preferred IP address for
targeted LDP, an LSR should attempt a targeted LDP session with the
Router ID [RFC2328] [RFC5305] [RFC5340], unless it is configured
otherwise.
No protection is available until the TLDP session has been
established and a label for the destination has been learned from the
remote LFA repair target. If for any reason the TLDP session cannot
not be established, an implementation SHOULD advise the operator
about the protection setup issue using any well known mechanism such
as Syslog [RFC5424] or SNMP [RFC3411].
8. Analysis of Real World Topologies
This section gives the results of analysing a number of real world
service provider topologies collected between October 2012 and the
date of this draft.
8.1. Topology Details
The figure below characterises each topology (topo) studied in terms
of :
o The number of nodes (# nodes) excluding pseudonodes.
o The number of bidirectional links ( # links) including parallel
links and links to and from pseudonodes.
o The number of node pairs that are connected by one or more links
(# pairs).
o The number of node pairs that are connected by more than one (i.e.
parallel) link ( # para).
o The number of links (excluding pseudonode links, which are by
definition asymmetric) that have asymmetric metrics (#asym).
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+------+---------+---------+---------+--------+--------+
| topo | # nodes | # links | # pairs | # para | # asym |
+------+---------+---------+---------+--------+--------+
| 1 | 315 | 570 | 560 | 10 | 3 |
| 2 | 158 | 373 | 312 | 33 | 0 |
| 3 | 655 | 1768 | 1314 | 275 | 1195 |
| 4 | 1281 | 2326 | 2248 | 70 | 10 |
| 5 | 364 | 811 | 659 | 80 | 86 |
| 6 | 114 | 318 | 197 | 101 | 4 |
| 7 | 55 | 237 | 159 | 67 | 2 |
| 8 | 779 | 1848 | 1441 | 199 | 437 |
| 9 | 263 | 482 | 413 | 41 | 12 |
| 10 | 86 | 375 | 145 | 64 | 22 |
| 11 | 162 | 1083 | 351 | 201 | 49 |
| 12 | 380 | 1174 | 763 | 231 | 0 |
| 13 | 1051 | 2087 | 2037 | 48 | 64 |
| 14 | 92 | 291 | 204 | 64 | 2 |
+------+---------+---------+---------+--------+--------+
8.2. LFA only
The figure below shows the percentage of protected destinations (%
prot) and percentage of guaranteed node protected destinations ( %
gtd N) for the set of topologies characterized in Section 8.1
achieved using only LFA repairs.
+------+--------+---------+
| topo | % prot | % gtd N |
+------+--------+---------+
| 1 | 78.5 | 36.9 |
| 2 | 97.3 | 52.4 |
| 3 | 99.3 | 58 |
| 4 | 83.1 | 63.1 |
| 5 | 99 | 59.1 |
| 6 | 86.4 | 21.4 |
| 7 | 93.9 | 35.4 |
| 8 | 95.3 | 48.1 |
| 9 | 82.2 | 49.5 |
| 10 | 98.5 | 14.9 |
| 11 | 99.6 | 24.8 |
| 12 | 99.5 | 62.4 |
| 13 | 92.4 | 51.6 |
| 14 | 99.3 | 48.6 |
+------+--------+---------+
8.3. RLFA
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The figure below shows the percentage of protected destinations (%
prot) and % guaranteed node protected destinations ( % gtd N) for
RLFA protection in the topologies studies. In addition, it show the
percentage of destinations using an RLFA repair (% PQ) together with
the total number of unidirectional RLFA targeted LDP session
established (# PQ), the number of PQ sessions which would be required
for complete protection, but which could not be established (no PQ).
It also shows the 50 (p50), 90 (p90) and 100 (p100) percentiles for
the number of individual LDP sessions terminating at an individual
node (whether used for TX, RX or both).
For example, if there were LDP sessions required A->B, A->C, C->A,
C->D, these would be counted as 2, 1, 2, 1 at nodes A,B,C and D
respectively because:-
A has two sessions (to nodes B and C)
B has one session (to node A)
C has two sessions (to nodes A and D)
D has one session (to node D)
In this study, remote LFA is only used when necessary. i.e. when
there is at least one destination which is not reparable by a per
destination LFA, and a single remote LFA tunnel is used (if
available) to repair traffic to all such destinations. The remote
LFA repair target points are computed using extended P space and
choosing the PQ node which has the lowest metric cost from the
repairing node.
+------+--------+--------+------+------+-------+-----+-----+------+
| topo | % prot |% gtd N | % PQ | # PQ | no PQ | p50 | p90 | p100 |
+------+--------+--------+------+------+-------+-----+-----+------+
| 1 | 99.7 | 53.3 | 21.2 | 295 | 3 | 1 | 5 | 14 |
| 2 | 97.5 | 52.4 | 0.2 | 7 | 40 | 0 | 0 | 2 |
| 3 | 99.999 | 58.4 | 0.7 | 63 | 5 | 0 | 1 | 5 |
| 4 | 99 | 74.8 | 16 | 1424 | 54 | 1 | 3 | 23 |
| 5 | 99.5 | 59.5 | 0.5 | 151 | 7 | 0 | 2 | 7 |
| 6 | 100 | 34.9 | 13.6 | 63 | 0 | 1 | 2 | 6 |
| 7 | 99.999 | 40.6 | 6.1 | 16 | 2 | 0 | 2 | 4 |
| 8 | 99.5 | 50.2 | 4.3 | 350 | 39 | 0 | 2 | 15 |
| 9 | 99.5 | 55 | 17.3 | 428 | 5 | 1 | 2 | 67 |
| 10 | 99.6 | 14.1 | 1 | 49 | 7 | 1 | 2 | 5 |
| 11 | 99.9 | 24.9 | 0.3 | 85 | 1 | 0 | 2 | 8 |
| 12 | 99.999 | 62.8 | 0.5 | 512 | 4 | 0 | 0 | 3 |
| 13 | 97.5 | 54.6 | 5.1 | 1188 | 95 | 0 | 2 | 27 |
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| 14 | 100 | 48.6 | 0.7 | 79 | 0 | 0 | 2 | 4 |
+------+--------+--------+------+------+-------+-----+-----+------+
Another study[ISOCORE2010] confirms the significant coverage increase
provided by Remote LFAs.
8.4. Comparison of LFA an RLFA results
The table below provides a side by side comparison the LFA and the
remote LFA results. This shows a significant improvement in the
percentage of protected destinations and normally a modest
improvement in the percentage of guaranteed node protected
destinations.
+------+--------+--------+---------+---------+
| topo | LFA | RLFA | LFA | RLFA |
| | % prot | %prot | % gtd N | % gtd N |
+------+--------+--------+---------+---------+
| 1 | 78.5 | 99.7 | 36.9 | 53.3 |
| 2 | 97.3 | 97.5 | 52.4 | 52.4 |
| 3 | 99.3 | 99.999 | 58 | 58.4 |
| 4 | 83.1 | 99 | 63.1 | 74.8 |
| 5 | 99 | 99.5 | 59.1 | 59.5 |
| 6 | 86.4 |100 | 21.4 | 34.9 |
| 7 | 93.9 | 99.999 | 35.4 | 40.6 |
| 8 | 95.3 | 99.5 | 48.1 | 50.2 |
| 9 | 82.2 | 99.5 | 49.5 | 55 |
| 10 | 98.5 | 99.6 | 14.9 | 14.1 |
| 11 | 99.6 | 99.9 | 24.8 | 24.9 |
| 12 | 99.5 | 99.999 | 62.4 | 62.8 |
| 13 | 92.4 | 97.5 | 51.6 | 54.6 |
| 14 | 99.3 |100 | 48.6 | 48.6 |
+------+--------+--------+---------+---------+
As shown in the table, remote LFA provides close to 100% prefix
protection against link failure in 11 of the 14 topologies studied,
and provides a significant improvement in two of the remaining three
cases. In an MPLS network, this is achieved without any saleability
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 explicitly-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
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described elsewhere in this document. Alternatively the segment
routing technology being defined in the IETF may be used to carry the
traffic between non-collocated P and Q nodes
[I-D.filsfils-rtgwg-segment-routing-use-cases],
[I-D.filsfils-rtgwg-segment-routing],
[I-D.gredler-rtgwg-igp-label-advertisement].
9. Management Considerations
The management of LFA and remote LFA is the subject of ongoing work
withing the IETF[I-D.ietf-rtgwg-lfa-manageability] to which the
reader is referred. Management considerations may lead to a
preference for the use of a remote LFA over an available LFA. This
preference is a matter for the network operator, and not a matter of
protocol correctness.
10. 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 [RFC6571], 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.
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.
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
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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 wish to thank Levente Csikor and Chris Bowers for their
contribution to the cost based algorithm text. We thank Stephane
Litkowski for his review of this work.
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-segment-routing-use-cases]
Filsfils, C., Francois, P., Previdi, S., Decraene, B.,
Litkowski, S., Horneffer, M., Milojevic, I., Shakir, R.,
Ytti, S., Henderickx, W., Tantsura, J., Kini, S., and E.
Crabbe, "Segment Routing Use Cases", draft-filsfils-rtgwg-
segment-routing-use-cases-02 (work in progress), October
2013.
[I-D.filsfils-rtgwg-segment-routing]
Filsfils, C., Previdi, S., Bashandy, A., Decraene, B.,
Litkowski, S., Horneffer, M., Milojevic, I., Shakir, R.,
Ytti, S., Henderickx, W., Tantsura, J., and E. Crabbe,
"Segment Routing Architecture", draft-filsfils-rtgwg-
segment-routing-01 (work in progress), October 2013.
[I-D.gredler-rtgwg-igp-label-advertisement]
Gredler, H., Amante, S., Scholl, T., and L. Jalil,
"Advertising MPLS labels in IGPs", draft-gredler-rtgwg-
igp-label-advertisement-05 (work in progress), May 2013.
[I-D.ietf-rtgwg-lfa-manageability]
Litkowski, S., Decraene, B., Filsfils, C., and K. Raza,
"Operational management of Loop Free Alternates", draft-
ietf-rtgwg-lfa-manageability-00 (work in progress), May
2013.
[I-D.psarkar-rtgwg-rlfa-node-protection]
psarkar@juniper.net, p., Gredler, H., Hegde, S.,
Raghuveer, H., cbowers@juniper.net, c., and S. Litkowski,
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"Remote-LFA Node Protection and Manageability", draft-
psarkar-rtgwg-rlfa-node-protection-02 (work in progress),
November 2013.
[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.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
[RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
Encoding", RFC 3032, January 2001.
[RFC3411] Harrington, D., Presuhn, R., and B. Wijnen, "An
Architecture for Describing Simple Network Management
Protocol (SNMP) Management Frameworks", STD 62, RFC 3411,
December 2002.
[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.
[RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic
Engineering", RFC 5305, October 2008.
[RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
for IPv6", RFC 5340, July 2008.
[RFC5424] Gerhards, R., "The Syslog Protocol", RFC 5424, March 2009.
[RFC5714] Shand, M. and S. Bryant, "IP Fast Reroute Framework", RFC
5714, January 2010.
[RFC6571] Filsfils, C., Francois, P., Shand, M., Decraene, B.,
Uttaro, J., Leymann, N., and M. Horneffer, "Loop-Free
Alternate (LFA) Applicability in Service Provider (SP)
Networks", RFC 6571, June 2012.
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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
Ning So
Tata Communications
Mobile Broadband Services
Email: Ning.So@tatacommunications.com
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