Internet DRAFT - draft-esale-mpls-ldp-node-frr
draft-esale-mpls-ldp-node-frr
MPLS Working Group Santosh Esale
INTERNET-DRAFT Raveendra Torvi
Intended Status: Proposed Standard Juniper Networks
Expires: September 14, 2017
Luyuan Fang
Microsoft
Luay Jalil
Verizon
March 13, 2017
Fast Reroute for Node Protection in LDP-based LSPs
draft-esale-mpls-ldp-node-frr-05
Abstract
This document describes procedures to support node protection for
unicast Label Switched Paths (LSPs) established by Label Distribution
Protocol (LDP). In order to protect a node N, the Point of Local
Repair (PLR) of N must discover the Merge Points (MPs) of node N such
that traffic can be redirected to them in case of node N failure.
Redirecting the traffic around the failed node N depends on existing
point-to-point LSPs originated from the PLR to the MPs while
bypassing the protected node N. The procedures described in this
document are topology independent in a sense that they provide node
protection in any topology so long as there is a alternate path in
the network that avoids the protected node.
Status of this Memo
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Copyright and License Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1 Abbreviations . . . . . . . . . . . . . . . . . . . . . . . 4
3. Merge Point (MP) Discovery . . . . . . . . . . . . . . . . . . 4
4. Constructing Bypass LSPs . . . . . . . . . . . . . . . . . . . 5
5. Obtaining Label Mapping from MP . . . . . . . . . . . . . . . . 6
6. Forwarding Considerations . . . . . . . . . . . . . . . . . . . 6
7. Synergy with node protection in mLDP . . . . . . . . . . . . . 7
8. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
11. Normative References . . . . . . . . . . . . . . . . . . . . . 7
12. Informative References . . . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
This document describes procedures to support node protection for
unicast Label Switched Paths (LSPs) established by Label Distribution
Protocol (LDP) [RFC5036]. In order to protect a node N, the Point of
Local Repair (PLR) of N must discover the Merge Points (MPs) of node
N such that traffic can be redirected to them in case of node N
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failure. Redirecting the traffic around the failed node N depends on
existing explicit path Point-to-Point (P2P) LSPs originated from the
PLR LSR to the MPs while bypassing node N. The procedures to setup
these P2P LSPs are outside the scope of this document, but one option
is to use RSVP-TE based techniques [RFC3209] to accomplish it.
Finally, sending traffic from the PLR to the MPs requires the PLR to
obtain FEC-label bindings from the MPs. The procedures described in
this document relies on Targeted LDP (tLDP) session [RFC5036] for the
PLR to obtain such FEC-Label bindings.
The procedure described in this document assumes the use of platform-
wide label space. The procedures for node protection described in
this document fall into the category of local protection. The
procedures described in this document apply to LDP LSPs bound to
either an IPv4 or IPv6 Prefix FEC element. The procedures described
in this document are topology independent in a sense that they
provide node protection in any topology so long as there is a
alternate path in the network that avoids the protected node. Thus
these procedures provide topology independent fast reroute.
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1.1 Abbreviations
PLR: Point of Local Repair - the LSR that redirects the traffic to
one or more Merge Point LSRs.
MP: Merge Point. Any LSR on the LDP-signaled (multi-point to
point) LSP, provided that the path from that LSR to the
egress of that LSP is not affected by the failure of the
protected node.
tLDP: A targeted LDP session is an LDP session between non-directly
connected LSRs, established using the LDP extended discovery
mechanism.
FEC: Forwarding equivalence class.
IGP: Interior Gateway Protocol.
BR: Border Router.
3. Merge Point (MP) Discovery
For a given LSP that traverses the PLR, the protected node N, and a
particular neighbor of the protected node, we'll refer to this
neighbor as the "next next-hop". Note that from the PLR's perspective
the protected node N is the next hop for the FEC associated with that
LSP. Likewise, from the protected node's perspective the next next-
hop is the next hop for that FEC. If for a given <LSP, PLR, N>
triplet the next next-hop is in the same routing subdomain (area) as
the PLR, then that next next-hop acts as the MP for that triplet. For
a given LSP traversing a PLR and the node protected by the PLR, the
PLR discovers its next next-hops (MPs) that are in the same routing
subdomain (IGP area) as the PLR from IGP shortest path first (SPF)
calculations. The discovery of next next-hop, depending on an
implementation, may not involve any additional SPF, above and beyond
what will be needed by either ISIS or OSPF anyway, as the next next-
hop, just like the next-hop, is a by-product of SPF computation.
Also, the PLR may discover all possible MPs from either its traffic
engineering database or link state database. Some implementations MAY
need appropriate configuration to populate the traffic engineering
database. The traffic engineering database is populated by routing
protocols such as ISIS and OSPF or configured statically.
If for a given <LSP, PLR, N> triplet the node protected by the PLR is
an Border Router (BR), then the PLR and the next next-hop may end up
in different routing subdomain. This could happen when an LSP
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traversing the PLR and the protected node does not terminate in the
same routing subdomain as the PLR. In this situation the PLR may not
be able to determine the next next-hop from shortest path first (SPF)
calculations, and thus may not be able to use the next next-hop as
the MP. In this scenario the PLR uses an "alternative" BR as the MP,
where an alternative BR is defined as follows. For a given LSP that
traverses the PLR and the (protected) BR, an alternative BR is
defined as any BR that advertises into PLR's own routing subdomain
reachability to the FEC associated with the LSP.
Note that even if a PLR protects an BR, for some of the LSPs
traversing the PLR and the BR, the next next-hops may be in the same
routing subdomain as the PLR, in which case these next next-hops act
as MPs for these LSPs. Note that even if the protected node is not an
BR, if an LSP traversing the PLR and the protected node does not
terminate in the same routing subdomain as the PLR, then for this LSP
the PLR MAY use an alternative BR (as defined earlier), rather than
the next next-hop as the MP. When there are several candidate BRs for
alternative BR, the LSR MUST select one BR. The algorithm used for
the alternative BR selection is a local matter but one option is to
select the BR per FEC based on shortest path from PLR to the BR.
4. Constructing Bypass LSPs
As mentioned before, redirecting traffic around the failed node N
depends on existing explicit path Point-to-Point (P2P) LSPs
originated from the PLR to the MPs while bypassing node N. Let's
refer to these LSPs as "bypass LSPs". While the procedures to signal
these bypass LSPs are outside the scope of this document, this
document assumes use of RSVP-TE LSPs [RFC3209] to accomplish it. Once
a PLR that protects a given node N discovers the set of MPs
associated with itself and the protected node, at the minimum the PLR
MUST (automatically) establish bypass LSPs to all these MPs. The
bypass LSPs MUST be established prior to the failure of the protected
node.
One could observe that if the protected node is not an BR and the PLR
does not use alternative BR(s) as MP(s), then the set of all the IGP
neighbors of the protected node forms a superset of the MPs. Thus it
would be sufficient for the PLR to establish bypass LSPs with all the
IGP neighbors of the protected node, even though some of these
neighbors may not be MPs for any of the LSPs traversing the PLR and
the protected node.
The bypass LSPs MUST avoid traversing the protected node, which means
that the bypass LSPs are explicitly routed LSPs. Of course, using
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RSVP-TE to establish bypass LSPs allows these LSPs to be explicitly
routed. As a given router may act as an MP for more than one LSP
traversing the PLR, the protected node, and the MP, the same bypass
LSP will be used to protect all those LSPs.
5. Obtaining Label Mapping from MP
As mentioned before, sending traffic from the PLR to the MPs requires
the PLR to obtain FEC-label bindings from the MPs. The solution
described in this document relies on Targeted LDP (tLDP) session
[RFC5036] for the PLR to obtain such mappings. Specifically, for a
given PLR and the node protected by this PLR, at the minimum the PLR
MUST (automatically) establish tLDP with all the MPs associated with
this PLR and the protected node. These tLDP sessions MUST be
established prior to the failure of the protected node. One could
observe that if the protected node is not an BR and the PLR does not
use alternative BR(s) as MP(s), then the set of all the IGP neighbors
of the protected node forms a superset of the MPs. Thus it will be
sufficient for the PLR to (automatically) establish tLDP session with
all the IGP neighbors of the protected node - except the PLR - that
are in the same area as the PLR, even though some of these neighbors
may not be MPs for any of the LSPs traversing the PLR and the
protected node.
At the minimum for a given tLDP peer the PLR MUST obtain FEC-label
mapping for the FEC(s) for which the peer acts as an MP. The PLR MUST
obtain this mapping before the failure of the protected node. To
obtain this mapping for only these FECs and no other FECs that the
peer may maintain, the PLR SHOULD rely on the LDP Downstream on
Demand (DoD) procedures [RFC5036]. Otherwise, without relying on the
DoD procedures, the PLR may end up receiving from a given tLDP peer
FEC-label mappings for all the FECs maintained by the peer, even if
the peer does not act as an MP for some of these FECs. If the LDP DoD
procedures are not used, then for the purpose of the procedures
specified in this draft the only label mappings that SHOULD be
exchanged are for the Prefix FEC elements whose PreLen value is
either 32 (IPv4), or 128 (IPv6); label mappings for the Prefix FEC
elements with any other PreLen value SHOULD NOT be exchanged.
When a PLR has one or more BRs acting as MPs, the PLR MAY use the
procedures specified in [draft-ietf-mpls-app-aware-tldp] to limit the
set of FEC-label mappings received from non-BR MPs to only the
mappings for the FECs associated with the LSPs that terminate in the
PLR's own routing subdomain (area).
6. Forwarding Considerations
When a PLR detects failure of the protected node then rather than
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swapping an incoming label with a label that the PLR received from
the protected node, the PLR swaps the incoming label with the label
that the PLR receives from the MP, and then pushes the label
associated with the bypass LSP to that MP.
To minimize micro-loop during the IGP global convergence PLR may
continue to use the bypass LSP during network convergence by adding
small delay before switching to a new path.
7. Synergy with node protection in mLDP
Both the bypass LSPs and tLDP sessions described in this document
could also be used for the purpose of mLDP node protection, as
described in [draft-ietf-mpls-mldp-node-protection].
8. Security Considerations
The same security considerations apply as those for the base LDP
specification, as described in [RFC5036].
9. IANA Considerations
This document introduces no new IANA Considerations.
10. Acknowledgements
We are indebted to Yakov Rekhter for many discussions on this topic.
We like to thank Hannes Gredler, Aman Kapoor, Minto Jeyananth, Eric
Rosen, Vladimir Blazhkun and Loa Andersson for through review of this
document.
11. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3209] D. Awduche, et al., "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC3209, Decembet 2001.
[RFC5036] Andersson, L., Minei, I., and B. Thomas, "LDP
Specification", RFC 5036, October 2007.
[draft-ietf-mpls-app-aware-tldp] Esale, S., et al.,"Application-
aware Targeted LDP", draft-esale-mpls-app-aware-tldp, work
in progress.
12. Informative References
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[RFC7715], IJ. Wijnands, et al., "Multipoint LDP (mLDP) Node
Protection", RFC7715, January 2016.
Authors' Addresses
Santosh Esale
Juniper Networks
EMail: sesale@juniper.net
Raveendra Torvi
Juniper Networks
EMail: rtorvi@juniper.net
Luyuan Fang
Microsoft
Email: lufang@microsoft.com
Luay Jalil
Verizon
Email: luay.jalil@verizon.com
Esale, et al. Expires September 14, 2017 [Page 8]
