Internet DRAFT - draft-shen-mpls-egress-protection-framework
draft-shen-mpls-egress-protection-framework
Internet Engineering Task Force Yimin Shen
Internet-Draft Minto Jeyananth
Intended status: Standards Track Juniper Networks
Expires: May 18, 2018 Bruno Decraene
Orange
Hannes Gredler
RtBrick Inc
Carsten Michel
Deutsche Telekom
Huaimo Chen
Yuanlong Jiang
Huawei Technologies Co., Ltd.
November 14, 2017
MPLS Egress Protection Framework
draft-shen-mpls-egress-protection-framework-07
Abstract
This document specifies a fast reroute framework for protecting IP/
MPLS services and MPLS transport tunnels against egress node and
egress link failures. In this framework, the penultimate-hop router
of an MPLS tunnel acts as the point of local repair (PLR) for egress
node failure, and the egress router of the MPLS tunnel acts as the
PLR for egress link failure. Each of them pre-establishes a bypass
tunnel to a protector. Upon an egress node or link failure, the
corresponding PLR performs local failure detection and local repair,
by rerouting packets over the corresponding bypass tunnel. The
protector in turn performs context label switching or context IP
forwarding to send the packets to the ultimate service
destination(s). This mechanism can be used to reduce traffic loss
before global repair reacts to the failure and control plane
protocols converge on the topology changes due to the failure. The
framework is applicable to all types of IP/MPLS services and MPLS
tunnels. Under the framework, service protocol extensions may be
further specified to support service label distribution to the
protector.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on May 18, 2018.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Specification of Requirements . . . . . . . . . . . . . . . . 5
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 7
5. Egress node protection . . . . . . . . . . . . . . . . . . . 8
5.1. Reference topology . . . . . . . . . . . . . . . . . . . 8
5.2. Egress node failure and detection . . . . . . . . . . . . 8
5.3. Protector and PLR . . . . . . . . . . . . . . . . . . . . 9
5.4. Protected egress . . . . . . . . . . . . . . . . . . . . 10
5.5. Egress-protected tunnel and service . . . . . . . . . . . 11
5.6. Egress-protection bypass tunnel . . . . . . . . . . . . . 11
5.7. Context ID, context label, and context based forwarding . 12
5.8. Advertisement and path resolution for context ID . . . . 14
5.9. Egress-protection bypass tunnel establishment . . . . . . 15
5.10. Local repair on PLR . . . . . . . . . . . . . . . . . . . 15
5.11. Service label distribution from egress router to
protector . . . . . . . . . . . . . . . . . . . . . . . . 16
5.12. Centralized protector mode . . . . . . . . . . . . . . . 16
6. Egress link protection . . . . . . . . . . . . . . . . . . . 18
7. Global repair . . . . . . . . . . . . . . . . . . . . . . . . 21
8. Example: Layer-3 VPN egress protection . . . . . . . . . . . 21
8.1. Egress node protection . . . . . . . . . . . . . . . . . 23
8.2. Egress link protection . . . . . . . . . . . . . . . . . 24
8.3. Global repair . . . . . . . . . . . . . . . . . . . . . . 24
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9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
10. Security Considerations . . . . . . . . . . . . . . . . . . . 24
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 25
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 25
12.1. Normative References . . . . . . . . . . . . . . . . . . 25
12.2. Informative References . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26
1. Introduction
In MPLS networks, label switched paths (LSPs) are widely used as
transport tunnels to carry IP and MPLS services across MPLS domains.
Examples of MPLS services are layer-2 VPNs, layer-3 VPNs,
hierarchical LSPs, and others. In general, a tunnel may carry
multiple services of one or multiple types, if the tunnel can satisfy
both individual and aggregate requirements (e.g. CoS, QoS) of these
services. The egress router of the tunnel should host the
corresponding service instances of the services. An MPLS service
instance is responsible for forwarding service packets via an egress
link to the service destination, based on a service label. An IP
service instance is responsible for doing the same based on a service
IP address. The egress link is often called a PE-CE (provider edge -
customer edge) link or attachment circuit (AC).
Today, local repair based fast reroute mechanisms [RFC4090],
[RFC5286], [RFC7490], [RFC7812] have been widely deployed to protect
MPLS tunnels against transit link/node failures. They can achieve
fast restoration of traffic in the order of tens of milliseconds.
Local repair refers to the scenario where the router upstream to an
anticipated failure (aka. PLR, i.e. point of local repair) pre-
establishes a bypass tunnel to the router downstream of the failure
(aka. MP, i.e. merge point), and pre-installs the forwarding state
of the bypass tunnel in the data plane. The PLR also uses a rapid
mechanism (e.g. link layer OAM, BFD, and others) to locally detect
the failure in the data plane. When the failure occurs, the PLR
reroutes traffic through the bypass tunnel to the MP, allowing the
traffic to continue to flow to the tunnel's egress router.
This document describes a fast reroute framework for egress node and
egress link protection. Similar to transit link/node protection,
this framework relies on a PLR to perform local failure detection and
local repair. In egress node protection, the PLR is the penultimate-
hop router of a tunnel. In egress link protection, the PLR is the
egress router of the tunnel. The framework relies on a so-called
"protector" to serve as the tailend of a bypass tunnel. The
protector is a router that hosts "protection service instances" and
has its own connectivity or paths to service destinations. When a
PLR is doing local repair, the protector is responsible for
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performing "context label switching" for rerouted MPLS service
packets and "context IP forwarding" for rerouted IP service packets.
Thus, the service packets can continue to reach service destinations
with minimum disruption.
This framework considers an egress node failure as a failure of a
tunnel, as well as a failure of all the services carried by the
tunnel, because service packets can no longer reach the service
instances on the egress router. Therefore, the framework addresses
egress node protection at both tunnel level and service level
simultaneously. Likewise, the framework considers an egress link
failure as a failure of all the services traversing the link, and
addresses egress link protection at the service level.
This framework requires that the destination (a CE or site) of a
service MUST be dual-homed or have dual paths to an MPLS network,
normally via two MPLS edge routers. One of them is the egress router
of the service's transport tunnel, and the other is a backup egress
router which hosts "backup service instances". In the "co-located"
protector mode in this document, the backup egress router serves as a
protector, and hence each backup service instance acts as a
protection instance. In the "centralized" protector mode
(Section 5.12), a protector and a backup egress router are decoupled,
and each protection service instance and its corresponding backup
service instance are hosted on separate routers.
The framework is described by mainly referring to P2P (point-to-
point) tunnels. However, it is equally applicable to P2MP (point-to-
multipoint), MP2P (multipoint-to-point) and MP2MP (multipoint-to-
multipoint) tunnels, when a sub-LSP can be viewed as a P2P tunnel.
The framework is a multi-service and multi-transport framework. It
assumes a generic model where each service is comprised of a common
set of components, including a service instance, a service label, and
a service label distribution protocol, and the service is transported
over an MPLS tunnel of any type. The framework also assumes service
labels to be downstream assigned, i.e. assigned by egress routers.
Therefore, the framework is generally applicable to most existing and
future services. Services which use upstream-assigned service labels
are out of scope of this document and left for further study.
The framework does not require extensions for the existing signaling
and label distribution protocols (e.g. RSVP, LDP, BGP, etc.) of MPLS
tunnels. It expects transport tunnels and bypass tunnels to be
established by using the generic mechanisms provided by the
protocols. On the other hand, it does not preclude future extensions
to the protocols which may facilitate the procedures. One example of
such extension is [RSVP-EP]. The framework may need extensions for
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IGPs and service label distribution protocols, to support protection
establishment and context label switching. This document provides
guidelines for these extensions, but the specific details SHOULD be
addressed in separate documents.
The framework is intended to complement control-plane convergence and
global repair, which are traditionally used to recover networks from
egress node and egress link failures. Control-plane convergence
relies on control protocols to react on the topology changes due to a
failure. Global repair relies an ingress router to remotely detect a
failure and switch traffic to an alternative path. An example of
global repair is the BGP Prefix Independent Convergence mechanism
[BGP-PIC] for BGP established services. Compared with these
mechanisms, this framework is considered as faster in traffic
restoration, due to the nature of local failure detection and local
repair. However, it is RECOMMENDED that the framework SHOULD be used
in conjunction with control-plane convergence or global repair, in
order to take the advantages of both approaches to achieve more
effective protection. That is, the framework provides fast and
temporary repair, and control-plane convergence or global repair
provides ultimate and permanent repair.
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 RFC2119.
3. Terminology
Egress router - A router at the egress endpoint of a tunnel. It
hosts service instances for all the services carried by the tunnel,
and has connectivity with the destinations of the services.
Egress node failure - A failure of an egress router.
Egress link failure - A failure of the egress link (e.g. PE-CE link,
attachment circuit) of a service.
Egress failure - An egress node failure or an egress link failure.
Egress-protected tunnel - A tunnel whose egress router is protected
by a mechanism according to this framework. The egress router is
hence called a protected egress router.
Egress-protected service - An IP or MPLS service which is carried by
an egress-protected tunnel, and hence protected by a mechanism
according to this framework.
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Backup egress router - Given an egress-protected tunnel and its
egress router, this is another router which has connectivity with all
or a subset of the destinations of the egress-protected services
carried by the egress-protected tunnel.
Backup service instance - A service instance which is hosted by a
backup egress router, and corresponding to an egress-protected
service on a protected egress router.
Protector - A role acted by a router as an alternate of a protected
egress router, to handle service packets in the event of an egress
failure. A protector may be physically co-located with or decoupled
from a backup egress router, depending on the co-located or
centralized protector mode.
Protection service instance - A service instance hosted by a
protector, corresponding to the service instance of an egress-
protected service on a protected egress router. A protection service
instance is a backup service instance, if the protector is co-located
with a backup egress router.
PLR - A router at the point of local repair. In egress node
protection, it is the penultimate-hop router on an egress-protected
tunnel. In egress link protection, it is the egress router of the
egress-protected tunnel.
Protected egress {E, P} - A virtual node consisting of an ordered
pair of egress router E and protector P. It serves as the virtual
destination of an egress-protected tunnel, and as the virtual
location of the egress-protected services carried by the tunnel.
Context identifier (ID) - A globally unique IP address assigned to a
protected egress {E, P}.
Context label - A non-reserved label assigned to a context ID by a
protector.
Egress-protection bypass tunnel - A tunnel used to reroute service
packets around an egress failure.
Co-located protector mode - The scenario where a protector and a
backup egress router are co-located as one router, and hence each
backup service instance serves as a protection service instance.
Centralized protector mode - The scenario where a protector is a
dedicated router, and is decoupled from backup egress routers.
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Context label switching - Label switching performed by a protector,
in the label space of an egress router indicated by a context label.
Context IP forwarding - IP forwarding performed by a protector, in
the IP address space of an egress router indicated by a context
label.
4. Requirements
This document considers the followings as the design requirements of
this egress protection framework.
o The framework must support P2P tunnels. It should equally support
P2MP, MP2P and MP2MP tunnels, by treating each sub-LSP as a P2P
tunnel.
o The framework must support multi-service and multi-transport
networks. It must accommodate existing and future signaling and
label-distribution protocols of tunnels and bypass tunnels,
including RSVP, LDP, BGP, IGP, segment routing, and others. It
must also accommodate existing and future IP/MPLS services,
including layer-2 VPNs, layer-3 VPNs, hierarchical LSP, and
others. It must provide a generic solution for environments where
different types of services and tunnels may co-exist.
o The framework must consider minimizing disruption during
deployment. It should only involve routers close to egress, and
be transparent to ingress routers and other transit routers.
o In egress node protection, for scalability and performance
reasons, a PLR must be agnostic to services and service labels,
like PLRs in transit link/node protection. It must maintain
bypass tunnels and bypass forwarding state on a per-transport-
tunnel basis, rather than per-service-destination or per-service-
label basis. It should also support bypass tunnel sharing between
transport tunnels.
o A PLR must be able to use its local visibility or information of
routing and/or TE topology to compute or resolve a path for a
bypass tunnel to a protector.
o A protector must be able to perform context label switching for
rerouted MPLS service packets, based on service label(s) assigned
by an egress router. It must be able to perform context IP
forwarding for rerouted IP service packets, in the public or
private IP address space used by an egress router.
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o The framework must be able to work seamlessly with transit link/
node protection mechanisms to achieve end-to-end coverage.
o The framework must be able to work in conjunction with global
repair and control plane convergence.
5. Egress node protection
5.1. Reference topology
This document refers to the following topology when describing the
procedures of egress node protection.
services 1, ..., N
=====================================> tunnel
I ------ R1 ------- PLR --------------- E ----
ingress penultimate-hop egress \
| . (primary \
| . service \
| . instances) \
| . \
| . \ service
| . destinations
| . / (CEs, sites)
| . /
| . bypass /
| . tunnel /
| . /
| ............... /
R2 --------------- P ----
protector
(protection
service
instances)
Figure 1
5.2. Egress node failure and detection
An egress node failure refers to the failure of an MPLS tunnel's
egress router. At the service level, it also means a service
instance failure for each IP/MPLS service carried by the tunnel.
Ideally, an egress node failure can be detected by an adjacent router
(i.e. PLR in this framework) using a node liveness detection
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mechanism, or based on a collective failure of all the links to that
node. However, the assumption is that the mechanisms SHOULD be
reasonably fast, i.e. faster than control plane failure detection and
remote failure detection. Otherwise, local repair will not be able
to provide much benefit compared to control plane convergence or
global repair. In general, the speed, accuracy, and reliability of a
mechanism are the key factors to decide its applicability in egress
node protection. This document provides the following guidelines in
this regard.
o If the PLR has a reasonably fast mechanism to detect and
differentiate a link failure (of the link between the PLR and the
egress node) and an egress node failure, it SHOULD set up both
link protection and egress node protection, and trigger one and
only one protection upon a corresponding failure.
o If the PLR has a fast mechanism to detect a link failure and an
egress node failure, but cannot distinguish them; Or, if the PLR
has a fast mechanism to detect a link failure only, but not an
egress node failure, the PLR has two options:
1. It MAY set up link protection only, and leave the egress node
failure to global repair and control plane convergence to
handle.
2. It MAY set up egress node protection only, and treat a link
failure as a trigger for the egress node protection. However,
the assumption is that treating a link failure as an egress
node failure MUST NOT have a negative impact on services.
Otherwise, it SHOULD adopt the previous option.
5.3. Protector and PLR
A router is assigned to the "protector" role to protect a tunnel and
the services carried by the tunnel against an egress node failure.
The protector is responsible for hosting a protection service
instance for each protected service, serving as the tailend of a
bypass tunnel, and performing context label switching and/or context
IP forwarding for rerouted service packets.
A tunnel can be protected by only one protector at a given time.
Multiple tunnels to a given egress router may be protected by a
common protector or different protectors. A protector may protect
multiple tunnels with a common egress router or different egress
routers.
For each tunnel, its penultimate-hop router acts as a PLR. The PLR
pre-establishes a bypass tunnel to the protector, and pre-installs
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bypass forwarding state in the data plane. Upon detection of an
egress node failure, the PLR reroutes all the service packets
received on the tunnel though the bypass tunnel to the protector.
For MPLS service packets, the PLR keeps service labels intact in the
packets. The protector in turn forwards the rerouted service packets
towards the ultimate service destinations. Specifically, it performs
context label switching for MPLS service packets, based on service
labels assigned by the protected egress router; It performs context
IP forwarding for IP service packets, based on their destination
addresses.
The protector MUST have its own connectivity with each service
destination, via a direct link or a multi-hop path, which MUST NOT
traverse the protected egress router or be affected by the egress
node failure. This also requires that each service destination MUST
be dual-homed or have dual paths to the egress router and a backup
egress router which serves as the protector. Each protection service
instance on the protector relies on such connectivity to set up
forwarding state for context label switching and/or context IP
forwarding.
5.4. Protected egress
This document introduces the notion of "protected egress" as a
virtual node consisting of the egress router E of a tunnel and a
protector P. It is denoted by an ordered pair of {E, P}, indicating
the primary-and-protector relationship between the two routers. It
serves as the virtual destination of the tunnel, and the virtual
location of service instances for the services carried by the tunnel.
The tunnel and services are considered as being "associated" with the
protected egress {E, P}.
A given egress router E may be the tailend of multiple tunnels. In
general, the tunnels may be protected by multiple protectors, e.g.
P1, P2, and so on, with each Pi protecting a subset of the tunnels.
Thus, these routers form multiple protected egresses, i.e. {E, P1} ,
{E, P2}, and so on. Each tunnel is associated with one and only one
protected egress {E, Pi}. All the services carried by the tunnel are
then automatically associated with the same protected egress {E, Pi}.
Conversely, a service associated with a protected egress {E, Pi} MUST
be carried by a tunnel associated with the protected egress {E, Pi}.
This mapping MUST be ensured by the ingress router of the tunnel and
the service (Section 5.5).
Two routers X and Y may be protectors for each other. In this case,
they form two distinct protected egresses {X, Y} and {Y, X}.
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5.5. Egress-protected tunnel and service
A tunnel, which is associated with a protected egress {E, P}, is
called an egress-protected tunnel. It is associated with one and
only one protected egress {E, P}. Multiple egress-protected tunnels
may be associated with a given protected egress {E, P}. In this case,
they share the common egress router and protector, but may or may not
share a common ingress router, or a common PLR (i.e. penultimate-hop
router).
An egress-protected tunnel is considered as logically "destined" for
its protected egress {E, P}. However, its path MUST be resolved and
established with E as the physical tailend.
A service, which is associated with a protected egress {E, P}, is
called an egress-protected service. The egress router E hosts the
primary instance of the service, and the protector P hosts the
protection instance of the service.
An egress-protected service is associated with one and only one
protected egress {E, P}. Multiple egress-protected services may be
associated with a given protected egress {E, P}. In this case, these
services share the common egress router and protector, but may or may
not share a common egress-protected tunnel or a common ingress
router.
An egress-protected service MUST be mapped to an egress-protected
tunnel by its ingress router, based on the common protected egress
{E, P} of the service and the tunnel. This is achieved by
introducing the notion of "context ID" for protected egress {E, P},
as described in (Section 5.7).
5.6. Egress-protection bypass tunnel
An egress-protected tunnel destined for a protected egress {E, P}
MUST have a bypass tunnel from its PLR to the protector P. This
bypass tunnel is called an egress-protection bypass tunnel. The
bypass tunnel is considered as logically "destined" for the protected
egress {E, P}. However, due to its bypass nature, it MUST be resolved
and established with P as the physical tailend and E as the node to
avoid. The bypass tunnel MUST have the property that it MUST NOT be
affected by any topology change caused by an egress node failure.
An egress-protection bypass tunnel is associated with one and only
one protected egress {E, P}. A PLR may share an egress-protection
bypass tunnel for multiple egress-protected tunnels associated with a
common protected egress {E, P}. For multiple egress-protected tunnels
associated with a common protected egress {E, P}, there may be one or
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multiple egress-protection bypass tunnels from one or multiple PLRs
to the protector P, depending on the paths of the egress-protected
tunnels.
5.7. Context ID, context label, and context based forwarding
In this framework, a globally unique IPv4/v6 address is assigned to a
protected egress {E, P} to serve as the identifier of the protected
egress {E, P}. It is called a "context ID" due to its specific usage
in context label switching and context IP forwarding on the
protector. It is an IP address that is logically owned by both the
egress router and the protector. For the egress node, it indicates
the protector. For the protector, it indicates the egress router,
particularly the egress router's forwarding context. For other
routers in the network, it is an address reachable via both the
egress router and the protector in the routing domain and the TE
domain (Section 5.8), similar to an anycast address.
The main purpose of a context ID is to coordinate ingress router,
egress router, PLR and protector in setting up egress protection.
Given an egress-protected service associated with a protected egress
{E, P}, its context ID is used as below:
o If the service is an MPLS service, when E distributes a service
label binding message to the ingress router, E attaches the
context ID to the message. If the service is an IP service, when
E advertises the service destination address to the ingress
router, E also attaches the context ID to the advertisement
message. How the context ID is encoded in the messages is a
choice of the service protocol, and may need protocol extensions
to define a "context ID" object.
o The ingress router uses the context ID as destination to establish
or resolve an egress-protected tunnel. The ingress router then
maps the service to the tunnel for transportation. In this
process, the special semantics of the context ID is transparent to
the ingress router. The ingress router only treats the context ID
as an IP address of E, and behaves in the same manner as in
establishing or resolving a regular transport tunnel, although the
end result is an egress-protected tunnel.
o The context ID is conveyed to the PLR by the signaling protocol of
the egress-protected tunnel, or learned by the PLR via an IGP
(i.e. OSPF or ISIS) or a topology-driven label distribution
protocol (e.g. LDP). The PLR uses the context ID as destination
to establish or resolve an egress-protection bypass tunnel to P
while avoiding E.
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o P maintains a dedicated label space or a dedicated IP address
space for E, depending on whether the service is MPLS or IP. This
is referred to as "E's label space" or "E's IP address space",
respectively. P uses the context ID to identify the space.
o If the service is an MPLS service, E also distributes the service
label binding message to P. This is the same label binding
message that E advertises to the ingress router, attached with the
context ID. Based on the context ID, P installs the service label
in an MPLS forwarding table corresponding to E's label space. If
the service is an IP service, P installs an IP route in an IP
forwarding table corresponding to E's IP address space. In either
case, the protection service instance on P interprets the service
and constructs forwarding state for the route based on P's own
connectivity to the service's destination.
o P assigns a non-reserved label to the context ID. In the data
plane, this label represents the context ID and indicates E's
label space and IP address space. Therefore, it is called a
"context label".
o The PLR may establish the egress-protection bypass tunnel to P in
several manners. If the bypass tunnel is established by RSVP, the
PLR signals the bypass tunnel with the context ID as destination,
and P binds the context label to the bypass tunnel. If the bypass
tunnel is established by LDP, P advertises the context label for
the context ID as an IP prefix FEC. If the bypass tunnel is
established by the PLR in a hierarchical manner, the PLR treats
the context label as a one-hop LSP over a regular bypass tunnel to
P (e.g. a bypass tunnel to P's loopback IP address). If the
bypass tunnel is constructed by using segment routing, the bypass
tunnel is represented by a stack of SID labels with the context
label as the inner-most SID label (Section 5.9). In any case, the
bypass tunnel is a UHP tunnel whose incoming label at P is the
context label.
o During local repair, all the service packets received by P on the
bypass tunnel have the context label as top label. P first pops
the context label. For an MPLS service packet, P further looks up
the service label in E's label space indicated by the context
label, which is called context label switching. For an IP service
packet, P looks up the IP destination address in E's IP address
space indicated by the context label, which is called context IP
forwarding.
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5.8. Advertisement and path resolution for context ID
Path resolution are computation for a context ID are done on ingress
routers for egress-protected tunnels, and on PLRs for egress-
protection bypass tunnels. Therefore, given a protected egress {E,
P} and its context ID, E and P MUST coordinate the context ID in the
routing domain and the TE domain via IGP advertisement. The context
ID MUST be advertised in such a manner that all egress-protected
tunnels MUST have E as tailend, and all egress-protection bypass
tunnels MUST have P as tailend while avoiding E.
This document suggests two approaches:
1. The first approach is called "proxy mode". It requires E and P,
but not the PLR, to have the knowledge of the egress protection
schema. E and P advertise the context ID as a virtual proxy node
(i.e. a logical node) connected to the two routers, with the link
between the proxy node and E having more preferable IGP and TE
metrics than the link between the proxy node and P. Therefore,
all egress-protected tunnels destined for the context ID should
automatically follow the shortest IGP or TE paths to E. Each PLR
will no longer view itself as a penultimate-hop, but rather two
hops away from the proxy node, via E. The PLR will be able to
find a bypass path via P to the proxy node, while the bypass
tunnel should actually be terminated by P.
2. The second approach is called "alias mode". It requires P and
the PLR, but not E, to have the knowledge of the egress
protection schema. E simply advertises the context ID as a
regular IP address. P advertises the context ID and the context
label by using a "context ID label binding" advertisement. The
advertisement MUST be understood by the PLR. In both routing
domain and TE domain, the context ID is only reachable via E.
This ensures that all egress-protected tunnels destined for the
context ID should have E as tailend. Based on the "context ID
label binding" advertisement, the PLR can establish an egress-
protection bypass tunnel in several manners (Section 5.9). The
"context ID label binding" advertisement is defined as IGP
mirroring context segment in [SR-ARCH], [SR-OSPF] and [SR-ISIS].
These IGP extensions are generic in nature, and hence can be used
for egress protection purposes.
In a scenario where an egress-protected tunnel is an inter-area or
inter-AS tunnel, its associated context ID MUST be propagated from
the residing area/AS to the other areas/AS' via IGP or BGP, so that
the ingress router of the tunnel can have the reachability to the
context ID. The propagation process of the context ID SHOULD be the
same as that of a regular IP address in an inter-area/AS environment.
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5.9. Egress-protection bypass tunnel establishment
A PLR MUST know the context ID of a protected egress {E, P} in order
to establish an egress-protection bypass tunnel. The information is
obtained from the signaling or label distribution protocol of the
egress-protected tunnel. The PLR may or may not need to have the
knowledge of the egress protection schema. All it does is to set up
a bypass tunnel to a context ID while avoiding the next-hop router
(i.e. egress router). This is achievable by using a constraint-based
computation algorithm similar to those which are commonly used in the
computation of traffic engineering paths and loop-free alternate
(LFA) paths. Since the context ID is advertised in the routing
domain and the TE domain by IGP according to Section 5.8, the PLR
should be able to resolve or establish such a bypass path with the
protector as tailend. In some cases like the proxy mode, the PLR may
do so in the same manner as transit node protection.
An egress-protection bypass tunnel may be established via several
methods:
(1) It may be established by a signaling protocol (e.g. RSVP), with
the context ID as destination. The protector binds the context label
to the bypass tunnel.
(2) It may be formed by a topology driven protocol (e.g. LDP with
various LFA mechanisms). The protector advertises the context ID as
an IP prefix FEC, with the context label bound to it.
(3) It may be constructed as a hierarchical tunnel. When the
protector uses the alias mode (Section 5.8), the PLR will have the
knowledge of the context ID, context label, and protector (i.e. the
advertiser). The PLR can then establish the bypass tunnel in a
hierarchical manner, with the context label as a one-hop LSP over a
regular bypass tunnel to the protector's IP address (e.g. loopback
address). This regular bypass tunnel may be established by RSVP,
LDP, segment routing, and others.
5.10. Local repair on PLR
In this framework, a PLR is agnostic to services and service labels.
This obviates the need to maintain bypass forwarding state on a per-
service basis, and allows bypass tunnel sharing between egress-
protected tunnels. The PLR may share an egress-protection bypass
tunnel for multiple egress-protected tunnels associated with a common
protected egress {E, P}. During local repair, the PLR reroutes all
service packets received on the egress-protected tunnels via the
egress-protection bypass tunnel. Service labels remain intact in
MPLS service packets.
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Label operation during the rerouting depends on the bypass tunnel's
characteristics. If the bypass tunnel is a single level tunnel, the
rerouting will involve swapping the incoming label of an egress-
protected tunnel to the outgoing label of the bypass tunnel. If the
bypass tunnel is a hierarchical tunnel, the rerouting will involve
swapping the incoming label of an egress-protected tunnel to a
context label, and pushing the outgoing label of a regular bypass
tunnel. If the bypass tunnel is constructed by segment routing, the
rerouting will involve swapping the incoming label of an egress-
protected tunnel to a context label, and pushing a stack of SID
labels of the bypass tunnel.
5.11. Service label distribution from egress router to protector
As mentioned in previous sections, when a protector receives a
rerouted MPLS service packet, it performs context label switching
based on the packet's service label which is assigned by the
corresponding egress router. In order to achieve this, the protector
MUST maintain such kind of service labels in dedicated label spaces
on a per protected egress {E, P} basis, i.e. one label space for each
egress router that it protects.
Also, there MUST be a service label distribution protocol session
between each egress router and the protector. Through this protocol,
the protector learns the label binding of each egress-protected
service. This is the same label binding that the egress router
advertises to the corresponding ingress router, attached with a
context ID. The corresponding protection service instance on the
protector recognizes the service, and resolves forwarding state based
on its own connectivity with the service's destination. It then
installs the service label with the forwarding state in the label
space of the egress router, which is indicated by the context ID
(i.e. context label).
Different service protocols may use different mechanisms for such
kind of label distribution. Specific protocol extensions may be
needed on a per-protocol basis or per-service-type basis. The
details of the extensions SHOULD be specified in separate documents.
As an example, RFC 8104 specifies the LDP extensions for pseudowire
services.
5.12. Centralized protector mode
In this framework, it is assumed that the service destination of an
egress-protected service MUST be dual-homed to two edge routers of an
MPLS network. One of them is the protected egress router, and the
other is a backup egress router. So far in this document, the
discussion has been focusing on the scenario where a protector and a
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backup egress router are co-located as one router. Therefore, the
number of protectors in a network is equal to the number of backup
egress routers. As another scenario, a network may assign a small
number of routers to serve as dedicated protectors, each protecting a
subset of egress routers. These protectors are called centralized
protectors.
Topologically, a centralized protector may be decoupled from all
backup egress routers, or it may be co-located with one backup egress
router while decoupled from the other backup egress routers. The
procedures in this section assume the scenario where a protector and
a backup egress router are decoupled.
services 1, ..., N
=====================================> tunnel
I ------ R1 ------- PLR --------------- E ----
ingress penultimate-hop egress \
| . (primary \
| . service \
| . instances) \
| . \
| . bypass \ service
R2 . tunnel destinations
| . / (CEs, sites)
| . /
| . /
| . /
| . tunnel /
| =============> /
P ---------------- E' ---
protector backup egress
(protection (backup
service service
instances) instances)
Figure 2
Like a co-located protector, a centralized protector hosts protection
service instances, receives rerouted service packets from PLRs, and
performs context label switching and/or context IP forwarding. For
each service, instead of sending service packets directly to the
service destination, the protector MUST send them via another
transport tunnel to the corresponding backup service instance on a
backup egress router. The backup service instance in turn forwards
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them to the service destination. Specifically, in the case of an
MPLS service, the protector MUST swap the service label in each
received service packet to the label of the backup service advertised
by the backup egress router, and then push the label (or label stack)
of the transport tunnel.
In order for a centralized protector to map an egress-protected MPLS
service to a service hosted on a backup egress router, there MUST be
a service label distribution protocol session between the backup
egress router and the protector. Through this session, the backup
egress router advertises the service label of the backup service,
attached with the FEC of the egress-protected service and the context
ID of the protected egress {E, P}. Based on this information, the
protector associates the egress-protected service with the backup
service, resolves or establishes a transport tunnel to the backup
egress router, and accordingly sets up forwarding state for the label
of the egress-protected service in the label space of the egress
router.
The service label which the backup egress router advertises to the
protector can be the same as the label which the backup egress router
advertises to the ingress router(s), if and only if the forwarding
state of the label does not direct service packets towards the
protected egress router. Otherwise, the label is not usable for
egress protection, because it will create a loop, which MUST be
avoided. In this case, the backup egress router MUST advertise a
unique service label for egress protection, and set its forwarding
state to use the backup egress router's connectivity with the service
destination.
6. Egress link protection
Egress link protection is achievable through procedures similar to
that of egress node protection. In normal situations, an egress
router forwards service packets to a service destination based on a
service label, whose forwarding state points to an egress link. In
egress link protection, the egress router acts as PLR, by performing
local failure detection and local repair. Specifically, the egress
router pre-establishes an egress-protection bypass tunnel to a
protector, and installs bypass forwarding state for the service
label, pointing to the bypass tunnel. During local repair, the
egress router reroutes service packets via the bypass tunnel to the
protector. The protector in turn forwards the packets to the service
destination (in the co-located protector mode, as shown in Figure-3),
or forwards the packets to a backup egress router (in the centralized
protector mode, as shown in Figure-4).
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service
=====================================> tunnel
I ------ R1 ------- R2 --------------- E ----
ingress | ............. egress \
| . PLR \
| . (primary \
| . service \
| . instance) \
| . \
| . bypass service
| . tunnel destination
| . / (CE, site)
| . /
| . /
| . /
| . /
| ............... /
R3 --------------- P ----
protector
(protection
service
instance)
Figure 3
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service
=====================================> tunnel
I ------ R1 ------- R2 --------------- E ----
ingress | ............. egress \
| . PLR \
| . (primary \
| . service \
| . instance) \
| . \
| . bypass service
| . tunnel destination
| . / (CE, site)
| . /
| . /
| . /
| . tunnel /
| =============> /
R3 --------------- P ----
protector backup egress
(protection (backup
service service
instance) instance)
Figure 4
There are two approaches to set up the bypass forwarding state on the
egress router, depending on whether the egress router knows the
service label advertised by the backup egress router. The difference
is that one approach requires the protector to perform context label
switching, and the other one does not. Both approaches are equally
supported by this framework, and may be used in parallel.
(1) The first approach applies when the egress router does not
know the service label advertised by the backup egress router. In
this case, the egress router sets up the bypass forwarding state
as a label push with the outgoing label of the egress-protection
bypass tunnel. Rerouted packets will have the egress router's
service label intact. Therefore, the protector MUST perform
context label switching, and the bypass tunnel MUST be destined
for the context ID of the {E, P} and established as described in
Section 5.9. This approach is consistent with egress node
protection. Hence, a protector can serve in egress node and
egress link protection in a consistent manner, and both the co-
located protector mode and the centralized protector mode may be
used (Figure-3 and Figure-4).
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(2) The second approach applies when the egress router knows the
service label advertised by the backup egress route, via a label
distribution protocol session. In this case, the backup egress
router serves as the protector for egress link protection,
regardless of the protector of egress node protection, which
should be the same router in the co-located protector mode but may
be a different router in the centralized protector mode. The
egress router sets up the bypass forwarding state as a label swap
from the incoming service label to the service label of the
protector, followed by a push with the outgoing label (or label
stack) of the egress link protection bypass tunnel. The bypass
tunnel is a regular tunnel destined for an IP address of the
protector, instead of the context ID of the {E, P}. The protector
simply forwards rerouted service packets based on its own service
label, rather than performing context label switching. With this
approach, only the co-located protector mode is applicable.
Note that for a bidirectional service, the physical link of an egress
link may carry service traffic bi-directionally. Therefore, an
egress link failure may simultaneously be an ingress link failure for
the traffic in the opposite direction. However, protection for
ingress link failure SHOULD be provided by a separate mechanism, and
hence is out of the scope of this document.
7. Global repair
This framework provides a fast but temporary repair for egress node
and egress link failures. For permanent repair, it is RECOMMENDED
that the traffic SHOULD be moved to an alternative tunnel or
alternative services which are fully functional. This is referred to
as global repair. Possible triggers of global repair include control
plane notifications of tunnel and service status, end-to-end OAM and
fault detection at tunnel or service levels, and others. The
alternative tunnel and services may be pre-established as standby, or
dynamically established as a result of the triggers or network
protocol convergence.
8. Example: Layer-3 VPN egress protection
This section shows an example of egress protection for a layer-3 VPN.
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---------- R1 ----------- PE2 -
/ (PLR) (PLR) \
( ) / | | ( )
( ) / | | ( )
( site 1 )-- PE1 < | R3 ( site 2 )
( ) \ | | ( )
( ) \ | | ( )
\ | | /
---------- R2 ----------- PE3 -
(protector)
Figure 5
In this example, the site 1 (subnet 203.0.113.192/26) of a given VPN
is attached to PE1, and site 2 (subnet 203.0.113.128/26) is dual-
homed to PE2 and PE3. PE2 is the primary PE for site 2, and PE3 is
the backup PE. Each PE hosts a VPN instance. R1 and R2 are transit
routers in the MPLS network. The network uses OSPF as routing
protocol, and RSVP-TE as tunnel signaling protocol. The PEs use BGP
to exchange VPN prefixes and VPN labels between each other.
Using the framework in this document, the network assigns PE3 to be a
protector for PE2 to protect the VPN traffic in the direction from
site 1 to site 2. This is the co-located protector mode. Hence, PE2
and PE3 form a protected egress {PE2, PE3}. A context ID 198.51.100.1
is assigned to the protected egress {PE2, PE3}. The VPN instance on
PE3 serves as a protection instance for the VPN instance on PE2. On
PE3, a context label 100 is assigned to the context ID, and a label
table pe2.mpls is created to represent PE2's label space. PE3
installs the label 100 in its default MPLS forwarding table, with
nexthop pointing to the label table pe2.mpls. PE2 and PE3 are
coordinated to use the proxy mode to advertise the context ID in the
routing domain and the TE domain.
PE2 uses per-VRF VPN label allocation mode. It assigns a single
label 9000 to the VRF of the VPN. For a given VPN prefix
203.0.113.128/26 in site 2, PE2 advertises it along with the label
9000 and other attributes to PE1 and PE3 via BGP. In particular, the
NEXT_HOP attribute is set to the context ID 198.51.100.1.
Similarly, PE3 also uses per-VRF VPN label allocation mode. It
assigns a single label 10000 to the VRF of the VPN. For the VPN
prefix 203.0.113.128/26 in site 2, PE3 advertises it along with the
label 10000 and other attributes to PE1 and PE2 via BGP. In
particular, the NEXT_HOP attribute is set to an IP address of PE3.
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Upon receipt and acceptance of the BGP advertisement, PE1 uses the
context ID 198.51.100.1 as destination to compute a TE path for an
egress-protected tunnel. The resulted path is PE1->R1->PE2. PE1
then uses RSVP to signal the tunnel, with the context ID 198.51.100.1
as destination, and with the "node protection desired" flag set in
the SESSION_ATTRIBUTE of RSVP Path message. Once the tunnel comes
up, PE1 maps the VPN prefix 203.0.113.128/26 to the tunnel and
installs a route for the prefix in the corresponding VRF. The
route's nexthop is a push with the VPN label 9000, followed by a push
with the outgoing label of the egress-protected tunnel.
Upon receipt of the above BGP advertisement from PE2, PE3 (i.e. the
protector) recognizes the context ID 198.51.100.1 in the NEXT_HOP
attribute, and installs a route for label 9000 in the label table
pe2.mpls. PE3 sets the route's nexthop to a "protection VRF". This
protection VRF contains IP routes corresponding to the IP prefixes in
the dual-homed site 2, including 203.0.113.128/26. The nexthops of
these routes MUST be based on PE3's connectivity with site 2, even if
this connectivity is not the best path in PE3's VRF due to metrics
(e.g. MED, local preference, etc.), and MUST NOT use any path
traversing PE2. Note that the protection VRF is a logical concept,
and it may simply be PE3's own VRF if the VRF satisfies the
requirement.
8.1. Egress node protection
R1, i.e. the penultimate-hop router of the egress-protected tunnel,
serves as the PLR for egress node protection. Based on the "node
protection desired" flag and the destination address (i.e. context ID
198.51.100.1) of the tunnel, R1 computes a bypass path to
198.51.100.1 while avoiding PE2. The resulted bypass path is
R1->R2->PE3. R1 then signals the path (i.e. egress-protection bypass
tunnel), with 198.51.100.1 as destination.
Upon receipt of an RSVP Path message of the egress-protection bypass
tunnel, PE3 recognizes the context ID 198.51.100.1 as the
destination, and hence responds with the context label 100 in an RSVP
Resv message.
After the egress-protection bypass tunnel comes up, R1 installs a
bypass nexthop for the egress-protected tunnel. The bypass nexthop
is a swap from the incoming label of the egress-protected tunnel to
the outgoing label of the egress-protection bypass tunnel.
When R1 detects a failure of PE2, it will invoke the above bypass
nexthop to reroute VPN service packets. The packets will have the
label of the bypass tunnel as outer label, and the VPN label 9000 as
inner label. When the packets arrive at PE3, they will have the
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context label 100 as outer label, and the VPN label 9000 as inner
label. The context label will first be popped, and then the VPN
label will be looked up in the label table pe2.mpls. The lookup will
cause the VPN label to be popped, and the IP packets will finally be
forwarded to site 2 based on the protection VRF.
8.2. Egress link protection
PE2 serves as the PLR for egress link protection. It has already
learned the VPN label 10000 from PE3, and hence it uses the approach
(2) described in Section 6 to set up bypass forwarding state. It
signals an egress-protection bypass tunnel to PE3, by using the path
PE2->R3->PE3, and PE3's IP address as destination. After the bypass
tunnel comes up, PE2 installs a bypass nexthop for the VPN label
9000. The bypass nexthop is a label swap from the incoming label
9000 to the VPN label 10000 of PE3, followed by a label push with the
outgoing label of the bypass tunnel.
When PE3 detects a failure of the egress link, it will invoke the
above bypass nexthop to reroute VPN service packets. The packets
will have the label of the bypass tunnel as outer label, and the VPN
label 10000 as inner label. When the packets arrive at PE3, the VPN
label 10000 will be popped, and the IP packets will be forwarded
based on the VRF indicated by on the VPN label 10000.
8.3. Global repair
Eventually, global repair will take effect, as control plane
protocols converge on the new topology. PE1 will choose PE3 as new
entrance to site 2. Before that happens, the VPN traffic has been
protected by the above local repair.
9. IANA Considerations
This document has no request for new IANA allocation.
10. Security Considerations
The framework in this document relies on fast reroute around a
network failure. Specifically, service traffic is temporarily
rerouted from a PLR to a protector. In the centralized protector
mode, the traffic is further rerouted from the protector to a backup
egress router. Such kind of fast reroute is planned and anticipated,
and hence it should not be viewed as a new security threat.
The framework requires a service label distribution protocol to run
between an egress router and a protector. The available security
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measures of the protocol MAY be used to achieve a secured session
between the two routers.
11. Acknowledgements
This document leverages work done by Yakov Rekhter, Kevin Wang and
Zhaohui Zhang on MPLS egress protection. Thanks to Alexander
Vainshtein, Rolf Winter, and Lizhong Jin for their valuable comments
that helped shape this document and improve its clarity.
12. References
12.1. Normative References
[SR-ARCH] Filsfils, C., Previdi, S., Decraene, B., Litkowski, S.,
and R. Shakir, "Segment Routing Architecture", draft-ietf-
spring-segment-routing (work in progress), 2017.
[SR-OSPF] Psenak, P., Previdi, S., Filsfils, C., Gredler, H.,
Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
Extensions for Segment Routing", draft-ietf-ospf-segment-
routing-extensions (work in progress), 2017.
[SR-ISIS] Previdi, S., Filsfils, C., Bashandy, A., Gredler, H.,
Litkowski, S., Decraene, B., and J. Tantsura, "IS-IS
Extensions for Segment Routing", draft-ietf-isis-segment-
routing-extensions (work in progress), 2017.
12.2. Informative References
[RFC4090] Pan, P., Ed., Swallow, G., Ed., and A. Atlas, Ed., "Fast
Reroute Extensions to RSVP-TE for LSP Tunnels", RFC 4090,
DOI 10.17487/RFC4090, May 2005,
<https://www.rfc-editor.org/info/rfc4090>.
[RFC5286] Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for
IP Fast Reroute: Loop-Free Alternates", RFC 5286,
DOI 10.17487/RFC5286, September 2008,
<https://www.rfc-editor.org/info/rfc5286>.
[RFC7490] Bryant, S., Filsfils, C., Previdi, S., Shand, M., and N.
So, "Remote Loop-Free Alternate (LFA) Fast Reroute (FRR)",
RFC 7490, DOI 10.17487/RFC7490, April 2015,
<https://www.rfc-editor.org/info/rfc7490>.
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[RFC7812] Atlas, A., Bowers, C., and G. Enyedi, "An Architecture for
IP/LDP Fast Reroute Using Maximally Redundant Trees (MRT-
FRR)", RFC 7812, DOI 10.17487/RFC7812, June 2016,
<https://www.rfc-editor.org/info/rfc7812>.
[RFC8104] Shen, Y., Aggarwal, R., Henderickx, W., and Y. Jiang,
"Pseudowire (PW) Endpoint Fast Failure Protection",
RFC 8104, DOI 10.17487/RFC8104, March 2017,
<https://www.rfc-editor.org/info/rfc8104>.
[BGP-PIC] Bashandy, P., Filsfils, C., and P. Mohapatra, "BGP Prefix
Independent Convergence", draft-ietf-rtgwg-bgp-pic-05.txt
(work in progress), 2017.
[RSVP-EP] Chen, H., Liu, A., Saad, T., Xu, F., Huang, L., and N. So,
"Extensions to RSVP-TE for LSP Egress Local Protection",
draft-ietf-teas-rsvp-egress-protection (work in progress),
2017.
Authors' Addresses
Yimin Shen
Juniper Networks
10 Technology Park Drive
Westford, MA 01886
USA
Phone: +1 9785890722
Email: yshen@juniper.net
Minto Jeyananth
Juniper Networks
1133 Innovation Way
Sunnyvale, CA 94089
USA
Phone: +1 4089367563
Email: minto@juniper.net
Bruno Decraene
Orange
Email: bruno.decraene@orange.com
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Hannes Gredler
RtBrick Inc
Email: hannes@rtbrick.com
Carsten Michel
Deutsche Telekom
Email: c.michel@telekom.de
Huaimo Chen
Huawei Technologies Co., Ltd.
Email: huaimo.chen@huawei.com
Yuanlong Jiang
Huawei Technologies Co., Ltd.
Bantian, Longgang district
Shenzhen 518129
China
Email: jiangyuanlong@huawei.com
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