Internet DRAFT - draft-litkowski-rtgwg-lfa-rsvpte-cooperation
draft-litkowski-rtgwg-lfa-rsvpte-cooperation
Routing Area Working Group S. Litkowski
Internet-Draft B. Decraene
Intended status: Standards Track Orange
Expires: February 20, 2014 C. FilsFils
K. Raza
Cisco Systems
August 19, 2013
Interactions between LFA and RSVP-TE
draft-litkowski-rtgwg-lfa-rsvpte-cooperation-02
Abstract
This document defines the behavior of a node supporting Loopfree
Alternates (LFA) when the node has established RSVP TE tunnels. It
first describes the decisions to be made by the LFA mechanism with
respect to the use of TE tunnels as LFA candidates. Second, it
discusses the use of RSVP TE tunnels as a way to complement the LFA
coverage, illustrating how these technologies can benefit from each
other.
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].
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
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This Internet-Draft will expire on February 20, 2014.
Copyright Notice
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Copyright (c) 2013 IETF Trust and the persons identified as the
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. LFA FRR and MPLS-TE interactions . . . . . . . . . . . . . . 3
2.1. Use case : using MPLS LSP as LFA candidates . . . . . . . 3
2.2. Specifications of interactions between LFA and TE LSP . . 4
2.2.1. Having both a physical interface and a TE tunnel
toward a LFA . . . . . . . . . . . . . . . . . . . . 4
2.2.2. TE ingress LSP as LFA candidate . . . . . . . . . . . 4
2.2.3. Independence between LFA and TE FRR . . . . . . . . . 5
3. Operational considerations . . . . . . . . . . . . . . . . . 7
3.1. Relevance of joint LFA FRR and RSVP-TE FRR deployments . 7
3.2. Extending LFA coverage using RSVP-TE tunnels . . . . . . 8
3.2.1. Creating multihop tunnel to extend topology . . . . . 8
3.2.2. Selecting multihop tunnels to extend topology . . . . 9
4. Security Considerations . . . . . . . . . . . . . . . . . . . 10
5. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 10
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1. Normative References . . . . . . . . . . . . . . . . . . 10
7.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
When a failure occurs in an IP network, the subsequent converge
process often leads to traffic disruption. Some mechanisms are
available to limit traffic disruptions by pre-computing alternate
paths and locally reroute over these as soon as the failure is
detected. Such techniques are commonly known as "protection
mechanisms". Currently, the protection mechanisms widely used in
Service Provider networks are RSVP-TE Fast Reroute [RFC4090] and Loop
Free Alternates [RFC5286]. RSVP-TE FRR permits full network coverage
but with a quite high complexity in terms of operation, as well as
potential scaling issues. On the other hand, LFA offer a very easy,
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manageable, and scalable mechanism, but does not provide full
coverage.
This document discusses how LFA and RSVP-TE should interact. It
first describes how an LFA implementation should deal with existing
RSVP TE tunnels established by the LFA node, as well as its behavior
with respect to established IGP Shortcut tunnels [RFC3906]. Second,
the document suggets the use of RSVP-TE tunnels to extend LFA
coverage, and discusses the management and operational aspects of
such a practice.
2. LFA FRR and MPLS-TE interactions
This section discusses the various interactions among LFA FRR and
MPLS-TE FRR. It starts with a simple example emphasizing the
benefits of jointly using of LFA-FRR and MPLS-TE FRR, and then
summarizes the requirements for the interactions between LFAs and
MPLS-FRR.
2.1. Use case : using MPLS LSP as LFA candidates
In some cases, typically in ring shapped parts of network topologies,
links cannot be protected by LFAs. In the following topology, from
the point of view of R5, LFAs are able to partiatlly protect (49% of
the destination routers) from the failure of R3, while the failure of
R4 is not covered at all.
(30 routers) --- R1 ----(30)---- R2 --- (50 routers)
| |
(5) (30)
| |
R3 R4 --- (20 routers)
| |
(10) (30)
| |
------- R5 ------
Figure 1
Many networks deploy MPLS tunnels for traffic engineering and
resiliency reasons. To extend its benefit, an LFA implementation
could take advantage of such existing MPLS tunnels. In the exemple
above, if R5 has established TE tunnels bypassing R4 and R3, these
could be considerd as LFA candidates respectively protecting links
from R5 to R4 and R3.
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In the following section, we provide a detailed summary of the
behavior to be applied by an LFA implementation which would consider
the existence of MPLS TE tunnels to improve its applicability. The
explicit configuration of such tunnels with the intent of improving
LFA applicability is discussed in later sections.
2.2. Specifications of interactions between LFA and TE LSP
Here we summarize the normative requirements for the interaction
between LFA FRR and MPLS TE tunnels.
2.2.1. Having both a physical interface and a TE tunnel toward a LFA
If a node S has both a physical interface and a TE tunnel to reach a
LFA, it SHOULD use the physical interface unless :
1. The tunnel has been explicitly configured as an LFA candidate.
2. The tunnel does not pass through the link subject to LFA
protection.
In other words, if a node S has an IGP/LDP forwarding entry F1 with
outgoing interface i1, and S 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.
2.2.2. TE ingress LSP as LFA candidate
A TE LSP can be used as a virtual interface to reach a LFA if
1. The TE tunnel has been configured to allow its use as an LFA
candidate.
2. The TE tunnel does not pass through the primary outgoing
interface of D.
This would permit to extend LFA coverage as described in
[I-D.ietf-rtgwg-remote-lfa], in a controlled fashioned, as the
tunnels used by the fast reroute mechanism are defined by
configuration.
In other words, if a node S has an IGP/LDP forwarding entry F1 with
outgoing interface i1 and S originates a TE tunnel T1 terminating at
node Y, then an implementation SHOULD support a local policy which
instructs node S to consider Y as a virtual neighbor and hence
include Y as part of the LFA FRR alternate computation. In such
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case, an implementation MUST not use Y as an LFA for F1 if T1's
outgoing interface is i1.
2.2.3. Independence between LFA and TE FRR
2.2.3.1. Tunnel head-end case
Similar requirements can be expressed for TE IGP shortcut tunnels.
-----C1 ---------
/ | \
/ | \
PE1 ---- C2 --- C3 ---- PE2
| /
PE3
Figure 2
PE to Cx metrics are 50, Cx to Cx are 1
A service provider is often providing traffic-engineered path for
specific customer traffic (L3VPN, PW ...) to ensure path diversity or
traffic constraints. In the diagram above, we consider a TE tunnel
T2 built on a non shortest path as follows : PE1->C2->C3->PE2 and IGP
shortcut is activated on PE1 to make traffic to PE2 using T2. Based
on operational feedback, some implementations prevent LFA computation
to run for an interface where a TE tunnel exists. In our example, if
LFA is activated on N, we would not be able to have a protection for
PE3 destination as a tunnel exists on the interface. This current
observed behavior leads to a very limited coverage for LFA. In the
other hand, it is important to keep protection mechanisms independant
as much as possible to keep implementation simple. We propose the
following approach :
o If an IP prefix is reachable through a TE tunnel, LFA must not
compute a protection for it.
o If an IP prefix is reachable through a native IP path, LFA MUST
compute a protection for it disregarding the presence of a tunnel
or not on the primary interface.
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In other words, if a node S 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.
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.
2.2.3.2. Tunnel midpoint case
-----C1 ---------
/ | \
/ | \
PE1 ---- C2 --- C3 ---- PE2
| /
PE3
Figure 3
PE to Cx metrics are 50, except PE3-C3 (60), Cx to Cx are 1
In the diagram above, we consider a TE tunnel T2 built on a non
shortest path as follows : PE1->C2->C3->PE2 and IGP shortcut is
activated on PE1 to make traffic to PE2 using T2. C2 is a TE tunnel
midpoint router. In terms of forwarding, C2 has a MPLS TE forwarding
entry for T2, as well as an IP forwarding entry to PE2. As explained
in previous sections, it would be too restrictive and would limit LFA
benefit on C2 if C2 would not be able to compute an LFA for the IP
forwarding entry to PE2 due to the presence of a transit tunnel.
We propose the following approach for a midpoint router of a TE
tunnel :
o MPLS TE forwarding entries MUST not be protected by LFA (if an
operator wants protection, TE FRR could be enabled).
o IP forwarding entries MUST be protected by LFA disregarding the
presence of a TE tunnel transiting through the primary interface
of the destination.
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In our example :
o MPLS TE forwarding entry for T2 (ending on PE2) would be protected
by TE-FRR (if enabled).
o IP forwarding entry for PE2 would be protected by LFA.
In case of failure of C2-C3 :
o traffic from PE1 to PE2 (encapsulated in T2), would be protected
by TE FRR.
o traffic from PE3 to PE2 (native IP), would be protected by LFA.
In other words, if a node S 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.
3. Operational considerations
In this section, we first discuss the benefit of considering a joint
deployment of LFA and MPLS tunnels to achieve resiliency. We then
discuss one approach aiming at defining MPLS tunnels for the purpose
of complementing LFA coverage.
3.1. Relevance of joint LFA FRR and RSVP-TE FRR deployments
This section describes the deployment scenarios where it can be
beneficial to jointly use LFAs and RSVP-TE FRR.
There are many networks where RSVP-TE is already deployed. The
deployment of RSVP-TE is typically for two main reasons :
o Traffic engineering : a provider wants to route some flows on some
specific paths using constraints;
o Traffic protection using Fast-reroute ability
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:
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o Should increase protection on network where FRR is not available
everywhere. Although it may not provide full coverage, it will
increase the protection significantly.
o May provide better protection in specific cases than RSVP-TE FRR
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, in
some parts of the network. The following sections discusses the use
of basic RSVP-TE tunnels to extend protection coverage.
3.2. Extending LFA coverage using RSVP-TE tunnels
We already have seen in previous sections that RSVP-TE tunnels could
be established by an operator to complement LFA coverage. The method
of tunnel placement depends on what type of protection (link or node)
is required, as well as on the set of destinations or network parts
which requires better protection than what LFA can provide.
3.2.1. Creating multihop tunnel to extend topology
To extend the coverage, the idea is to use a mechanism extending LFA
by turning TE tunnels into LFA candidates. This mechanism is of a
local significance only.
When explicitly establishing tunnels for that purpose, choices have
to be made for the endpoints of such tunnels, in order to maximize
coverage while preserving management simplicity. Requirements are
that:
o Endpoints must satisfy equations from [RFC5286], otherwise it will
not be a valide LFA candidate: so when releasing traffic from
tunnel, the traffic will go to the destination without flowing
through the protected link or node. Depending on which equations
are satisfied, node or link protection will be provided by the
tunnel hop.
o Tunnel must not flow though the link or node to be protected,
explicit routing of tunnel is recommended to enforce this
condition.
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:
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o Non null intersection of Extended P-Space and Q-Space
o Computation of PQ node only for the remote end of the link
Based on this, [I-D.ietf-rtgwg-remote-lfa] may:
o Not find a tunnel endpoint;
o Not provide the more efficient protection : -- i.e. provides only
link protection, while there is node protection possible for a
specific destination
The proposed solution of manual explicitly routed tunnels is a good
complement for [I-D.ietf-rtgwg-remote-lfa] and provides more
flexibility:
o Always a possibility to find a tunnel endpoint for a specific
destination.
o Possibility to provide a better protection type (link vs. node).
3.2.2. Selecting multihop tunnels to extend topology
From a manageability point of view, computing a best Q node for each
destination could lead to have one different Q node for each
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 performing the
following computations per link:
Step 1 : Compute for each not covered destination (routed on the
link) the list of endpoints that are satisfying equations from
[RFC5286] (node or link protection equations depending of required
level of protection) : nodes in Q-Space
Step 2 : Remove endpoints that are not eligible for repair (Edge
nodes, low bandwidth meshed nodes, number of hops ...) : multiple
attributes could be specified to exclude some nodes from Q-Space :
The example of attributes include router type, metric to node,
bandwidth, packet loss, RTD ...
Step 3 : Within the list of endpoints (one list per destination),
order the endpoints by number of destination covered
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Step 4 : Choose the endpoint that has the highest number of
destination covered : some other criteria could be used to prefer
an endpoint from another (same type of criteria that excluded some
nodes from Q-Space)
Step 5 : Remove destinations covered by this endpoint from non
covered list
Step 6 : If non covered list is not empty, restart from Step 1
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.
4. Security Considerations
TBD.
5. Contributors
Significant contribution was made by Pierre Francois which the
authors would like to acknowledge.
6. IANA Considerations
This document has no actions for IANA.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.
[RFC5286] Atlas, A. and A. Zinin, "Basic Specification for IP Fast
Reroute: Loop-Free Alternates", RFC 5286, September 2008.
7.2. Informative References
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[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.ietf-rtgwg-remote-lfa]
Bryant, S., Filsfils, C., Previdi, S., Shand, M., and S.
Ning, "Remote LFA FRR", draft-ietf-rtgwg-remote-lfa-02
(work in progress), May 2013.
Authors' Addresses
Stephane Litkowski
Orange
Email: stephane.litkowski@orange.com
Bruno Decraene
Orange
Email: bruno.decraene@orange.com
Clarence FilsFils
Cisco Systems
Email: cfilsfil@cisco.com
Kamran Raza
Cisco Systems
Email: skraza@cisco.com
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