Abstract
This document specifies a mechanism for protecting RFC2547 based VPN
service against egress node failure. The mechanism enables local
repair to be performed immediately upon a egress node failure. In
particular, the router at point of local repair (PLR) can redirect
VPN traffic to a protector to repair in the order of tens of
milliseconds, achieving fast protection that is comparable to MPLS
fast reroute.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Specification of Requirements . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Reference topology . . . . . . . . . . . . . . . . . . . . . . 4
5. Theory of Operation . . . . . . . . . . . . . . . . . . . . . 5
5.1. Protector and Protection Models . . . . . . . . . . . . . 6
5.1.1. Co-located protector . . . . . . . . . . . . . . . . . 6
5.1.2. Centralized protector . . . . . . . . . . . . . . . . 6
5.2. Context Identifier and VPN prefixes. . . . . . . . . . . . 6
5.3. Forwarding State on Protector PE . . . . . . . . . . . . . 7
5.3.1. Alternate egress PE for protected prefix. . . . . . . 8
5.4. MPLS egress Fast reroute . . . . . . . . . . . . . . . . . 8
5.4.1. RSVP . . . . . . . . . . . . . . . . . . . . . . . . . 8
5.4.2. LDP . . . . . . . . . . . . . . . . . . . . . . . . . 8
6. Egress node Failure . . . . . . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
9.1. Normative References . . . . . . . . . . . . . . . . . . . 9
9.2. Informative References . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
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1. Introduction
This document specifies a mechanism for protecting RFC2547 based VPN
against egress PE failure. The procedures in this document are
relevant only when a VPN site is multi-homed to two or more PEs.
This is designed on the basis of MPLS context specific label
switching [RFC 5331]. Fast-protection refers to the ability to
provide local repair upon a failure in the order of tens of
milliseconds, which is comparable to MPLS fast-reroute [RFC 4090].
This is achieved by establishing local protection as close to a
failure as possible. Compared with the existing global repair
mechanisms that rely on control plane convergence, these procedures
can provide faster restoration for VPN traffic. However, they are
intended to complement the global repair mechanisms, rather than
replacing them in any way.
2. Specification of Requirements
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 RFC 2119.
3. Terminology
Protected PE: A PE which request protection for minimum one VPN
prefix.
Protected prefix: A VPN prefix that required protection in case of
Protected PE goes down.
Protector: A router which protect one or more VPN prefix when a
Protected PE goes down.
BGP nexthop: A nexthop advertised in the BGP-Update for the VPN
prefix by a BGP speaker.
VPN label: A label advertised by a BGP speaker for set of VPN
prefixes. This label can be per-VRF label or per nexthop label or
per prefix label.
Transport LSP: A LSP setup to BGP nexthop either by LDP or RSVP.
Alternative egress PE: A PE originates same IP prefix as Protected
prefix in a same VPN.
VPN transport LSP: A Transport LSP that carries VPN traffic.
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Context table: A context-specific label space routing table. This
table is per is populated with VPN labels advertised by the
protected-PE.
Context node: A stub router advertised into IGP by protected PE for a
context-identifier.
4. Reference topology
This document refers to the following topologies to describe various
roles and solution.
.......................
. .
+-------+--CE1----PE1 PE4----CE5---+-------+
| red | . \ / . | red |
| site1 | . \ / . | site2 |
+-------+--CE2-----+ P--P--PLR1 +----CE6---+-------+
. | / | | \ | .
. PE2 RR | PE5 .
. | \ | | / | .
+-------+--CE3-----+ P--P--PLR2 +----CE7--+-------+
| blue | . / \ . |blue |
| site1 | . / \ . |site2 |
+-------+--CE4-----PE3 PE6----CE8--+-------+
. .
. .
.......................
Figure 1
In Topology-1 two VPNs red and blue with two sites multihomed with
PEs. Let assume blue and red VPN site2 prefixes required egress
protection in case of PE5 goes down. PE5 is protected PE for site2
prefixes for both VPN. PE4 is alternate PE for red site2 prefixes.
PE6 is alternate PE for blue site2 prefixes. For PE4 could act as
protector for red VPN site2 and PE6 could acts as protector for blue
VPN site2. This model is co-located protector model. RR could act
as protector for both red and blue VPN site2. This is Centralized
protector model (A PE protecting set of VPNs and not connected to any
VPN site).
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.......................
. .
+-------+--CE1----PE1 PE4----CE5---+-------+
| red | . \ / . | red |
| site1 | . \ / . | site2 |
+-------+--CE2-----+ P--P--PLR1 +----CE6---+-------+
. | / | | \ | .
. PE2 RR | PE5 .
. | \ | | / | .
+-------+--CE3-----+ P--P--PLR2 +----CE7--+-------+
| red | . / \ . |red |
| site5 | . / \ . |site4 |
+-------+--CE4-----PE3 PE6----CE8--+-------+
. .
. .
.......................
Figure 2
In Topology-2 has a VPNs red with four sites and multihomed with PEs.
Let assume red VPN site2 and site4 prefixes required egress
protection in case of PE5 goes down. PE5 is protected PE for site2,
site4 prefixes for red VPN. PE4 is alternate PE for site2 prefixes.
PE6 is alternate PE for site4 prefixes. Either PE4 or PE6 could act
as protector. This is a slight variation of the co-located model.
5. Theory of Operation
The Egress PEs attached to multi-homed site export VPN prefixes with
different route distinguisher, different nexthop but with same route
target. The other PEs attached to other sites with same VPN import
these route into VRF creates more than one path to multi-homed sites.
When one egress PE goes down all VPN traffic towards the multihomed
site moved to alternate egress PEs attached to the multi-homed site
and this is done by ingress PE. The VPN traffic going via failed PE
get dropped in penultimate hop router until ingress PE reroute VPN
traffic. Even though connectivity of multi-homed site is not bound
to an egress PE the transport LSP bind to egress PE. As result of
down transport LSP VPN traffic getting dropped in P router. This
document specifies a mechanism that repair VPN traffic at point of
failure (typically a P router which penultimate hop of the transport
LSP) and still keep P router unaware of the VPN information with the
help of protector (a new role). The PLR (point of local repair) send
VPN traffic to protector through bypass LSP incase of egress PE
failure. This protector send VPN traffic received from PLR to the
alternative egress PE until the ingress reroute traffic to alternate
egress PE.
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5.1. Protector and Protection Models
Protector, is a new role for the egress PE failure local repair.
This protector role could be played by a PE(alternate egress PE) or
any other nodes which participates in VPN control plane for VPN
prefixes that required egress node protection. Hence, there are two
protection models based on the location and role of a protector.
5.1.1. Co-located protector
In this model, the protector is a alternate egress PE for a protected
prefix. It is co-located with the alternate PE for the protected
prefix, and it has a direct connection to the multi-homed site that
originate the protected prefix. In the event of an egress node
failure, the protector receives traffic from the PLR, and sends the
traffic to the multi-homed site. In the topology-1 PE4 could act as
protector for red VPN site2 and PE6 could acts as protector for blue
VPN site2. This model is co-located protector model. RR could act
as protector for both red and blue VPN site2. This is Centralized
protector model (A PE protecting set of VPNs and not connected to any
VPN site).
A slight variant of this model is, protector is not alternate PE for
a protected prefix but has same VRF. In the topology-2 either PE4 or
PE6 could act as protector. This is example for the above model.
5.1.2. Centralized protector
In this model, the protector serves as a centralized protector MAY
NOT have a direct connection to multi-homed site. This model can be
played by existing PEs or other PEs. In the event of an egress PE
failure, protector MUST send the traffic to a alternate egress PE
with VPN label advertised alternate egress PE for the prefix which in
turn sends the traffic to the multi-homed site. In the topology-1 RR
could act as protector for both red and blue VPN site2. This is
Centralized protector model (A PE protecting set of VPNs and not
connected to any VPN site).
A network MAY use either protection model or a combination of both,
depending on requirements.
5.2. Context Identifier and VPN prefixes.
The context-identifier is an IP address that is either globally
unique or unique in the private address space of the routing domain.
In Egress PE each VPN prefix is assigned to context-identifier. The
granularity of a context identifier is {Egress PE, VPN prefix} tuple.
However, a given context identifier MAY be assigned to one or
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multiple VPN prefixes.
Possible context identifier assignments are
o Unique context-identifier for all VPN prefixes, both VPN-IPv4 and
VPN-IPv6.
o Unique context-identifier per address family.
o Unique context-identifier per site for all VPN prefixes, both VPN-
IPv4 and VPN-IPv6.
o Unique context-identifier per site per address family.
o Unique context-identifier per CE address (nexthop).
o Unique context identifier for each prefix.
The first one is coarsest granularity of a context identifier and the
last one is finest granularity of a context identifier. While all of
the above options are possible in principle, their practical usage is
likely to vary widely, as not all of them may be of practical usage.
A given context identifier MUST NOT be used by more than one
protected PE. The egress PE that required protection for a VPN
prefix MUST set context-identifier as nexthop for VPN-IPv4 and IPv4-
Mapped context-identifier for VPN-IPv6 in BGP update. This context-
identifier as nexthop indicates to protector that this prefix need
protection. For e.g. In topology 1 PE5(protected PE) advertise VPN
prefixes with context-identifier as BGP nexthop.
5.3. Forwarding State on Protector PE
A protector maintain the forwarding state in context-specific label
spaces on a per protected PE basis. In particular, the protector
MUST learn the VPN label by participating the VPN routing and also
MUST keep all routes associated with VPN it required to protect.
For each VPN label with an associated context-identifier protector
MUST map the context identifier to a context-specific label space
[RFC 5331], and program the VPN label in that label space in
forwarding plane. For each VPN prefix that required protection
programmed in the forwarding plane with BGP nexthop to alternate
egress PE. This VPN label in the context-specific label space
identify the layer-3 forwarding table that need to lookup to send it
alternate egress PE. The protector MAY maintain only VPN prefix
originated with-in the multi-homed site for given {egress PE, VPN}.
These VPN labels in context table and VPN context table will not be
used in forwarding after ingress reroute the traffic to alternative
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PE. Protector MUST delete VPN label and the VPN context table after
ingress reroute the traffic. This shall be achieved with a timer.
This timer default value is 180 seconds.
5.3.1. Alternate egress PE for protected prefix.
Any route with BGP nexthop which has the following properties
Exact matching route-target set (RD may be different)
Exact matching Prefix part (excluding the RD)
will be eligible as alternate egress PE for prefix.
5.4. MPLS egress Fast reroute
A Protector should able to receive the traffic from PLR in the event
of an egress PE failure with forwarding context that enable protector
to repair the traffic.
5.4.1. RSVP
If RSVP LSP is used for transport then protector, primary and PLR
MUST follow backup egress procedure specified in [rsvp-egress-frr].
So that P router redirects the traffic during primary PE failure.
5.4.2. LDP
If it is LDP LSP then LDP FEC for this LSP MUST be the context
identifier of the protected segment. Prefix LFA or remote prefix LFA
with node protection can be used for redirect traffic to protector.
6. Egress node Failure
This section summarizes the procedures egress protection described
above section for completeness. A Egress PE, Protector, PLR follows
the methods described in [rsvp-egress-frr]. The protector programs
forwarding state in such a way that packets received on the bypass
LSP will be forwarded based on VPN label in the context table, and
prefix lookup in VPN context table. The context table identified by
the UHP label of the bypass LSP, i.e. the context identifier.
When the penultimate Hop router receives a VPN packet from the MPLS
network, if the egress PE is down, the PLR tunnels the packet through
the bypass LSP to the protector. The protector PE identifies the
forwarding context of the egress PE based on the top label of the
packet which is the UHP label of the bypass LSP. Then forwards
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protector the packet based on a second label lookup in the protected
PE's context label space followed by layer-3 lookup in the VPN
context table. These UHP label, context table label and layer-3
lookup results in forwarding the packet to the site or send it to
alternate egress PE based on protector model.
For E.g. In topology-1 RR is act as Protector and PE5 required
protection for red, blue site2 prefixes. As red, blue site2 VPN
prefixes advertised with context-identifier, the protector set up the
forwarding table for prefixes from site2 with alternative egress PE
as nexthop. When PLR detects PE5 failure it send to protector
through bypass LSP. In protector the top label identify the context
space table. VPN label in the context table identify the VPN layer-3
forwarding table with contains site2 prefixes with alternate PE as
nexthop. A Layer-3 lookup gives mpls path to alternate egress PE and
protector forward packet to alternate egress PE and reach to the
site2.
7. Security Considerations
The security considerations discussed in RFC 5036, RFC 5331, RFC
3209, and RFC 4090 apply to this document.
8. Acknowledgements
This draft is based on the ideas originally developed by JL Le Roux,
Bruno Decraene and Zubair Ahmad. This document leverages work done
by Yakov Rekhter and several others on LSP tail-end protection.
Thanks to Nischal Sheth, Nitin Bahadur, Yimin shen and Maciek
Konstantynowicz for their contribution.
9. References
9.1. Normative References
[RFC5331] Aggarwal, R., Rekhter, Y., and E. Rosen, "MPLS Upstream
Label Assignment and Context-Specific Label Space",
RFC 5331, August 2008.
[RFC4364] Rekhter, Y. and E. Rosen, "BGP/MPLS IP Virtual Private
Networks (VPNs)", RFC 4364, February 2006.
[RFC5036] Andersson, L., Minei, I., and B. Thomas, "LDP
Specification", RFC 5036, October 2007.
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[RFC2205] Braden, B., Zhang, L., Berson, S., Herzog, S., and S.
Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
Functional Specification", RFC 2205, September 1997.
[RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC 3209, December 2001.
[RFC4090] Pan, P., Swallow, G., and A. Atlas, "Fast Reroute
Extensions to RSVP-TE for LSP Tunnels", RFC 4090,
May 2005.
[RFC3471] Berger, L., "Generalized Multi-Protocol Label Switching
(GMPLS) Signaling Functional Description", RFC 3471,
January 2003.
[RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
Label Switching Architecture", RFC 3031, January 2001.
[LDP-UPSTREAM]
Aggarwal, R. and J. Roux, "MPLS Upstream Label Assignment
for LDP", draft-ietf-mpls-ldp-upstream (work in progress),
2011.
[RSVP-NON-PHP-OOB]
Ali, A., Swallow, Z., and R. Aggarwal, "Non PHP Behavior
and out-of-band mapping for RSVP-TE LSPs",
draft-ietf-mpls-rsvp-te-no-php-oob-mapping (work in
progress), 2011.
9.2. Informative References
[RFC5920] Fang, L., "Security Framework for MPLS and GMPLS
Networks", RFC 5920, July 2010.
[RFC5286] Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for
IP Fast Reroute: Loop-Free Alternates", RFC 5920,
September 2008.
[RFC5714] Shand, M. and S. Bryant, "IP Fast Reroute Framework",
RFC 5714, January 2010.
[rsvp-egress-frr]
Jeganathan, J., Gredler, H., and Y. Shen, "IP Fast Reroute
Framework", draft-minto-rsvp-lsp-egress-fast-protection-01
(work in progress), Oct 2012,
.
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Authors'
Jeyananth Minto Jeganathan
Juniper Networks
Hannes Gredler
Juniper Networks
Additional Source: Internet Engineering Task Force
