wdiff rfc8104.original rfc8104.txt

Internet Engineering Task Force Yimin (IETF) Y. Shen
Internet-Draft
Request for Comments: 8104 Juniper Networks
Intended status:
Category: Standards Track Rahul R. Aggarwal
Expires: July 7, 2017
ISSN: 2070-1721 Arktan, Inc
Wim Inc.
W. Henderickx
Nokia
Yuanlong
Y. Jiang
Huawei Technologies
January 3, Co., Ltd.
March 2017

Pseudowire (PW) Endpoint Fast Failure Protection
draft-ietf-pals-endpoint-fast-protection-05

Abstract

This document specifies a fast mechanism for protecting pseudowires
(PWs) transported by IP/MPLS tunnels against egress endpoint
failures, including egress AC (attachment circuit) attachment circuit (AC) failure, egress PE
(provider edge)
provider edge (PE) failure, multi-segment PW terminating PE failure,
and multi-segment PW switching PE failure. Operating on the basis of
multi-homed CE (customer edge),
multihomed customer edge (CE), redundant PWs, upstream label
assignment
assignment, and context specific context-specific label switching, the mechanism
enables local repair to be performed by the router upstream adjacent
to a failure. The router can restore a PW in the order of tens of
milliseconds, by rerouting traffic around the failure to a protector
through a pre-established bypass tunnel. Therefore, the mechanism
can be used to reduce traffic loss before global repair reacts to the
failure and the network converges on the topology changes due to the
failure.

Status of This Memo

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This Internet-Draft will expire on July 7, 2017.
http://www.rfc-editor.org/info/rfc8104.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Specification of Requirements . . . . . . . . . . . . . . . . 4
3. Reference Models for Egress Endpoint Failures . . . . . . . . 4
3.1. Single-Segment PW . . . . . . . . . . . . . . . . . . . . 4
3.2. Multi-Segment PW . . . . . . . . . . . . . . . . . . . . 8
4. Theory of Operation . . . . . . . . . . . . . . . . . . . . . 9
4.1. Applicability . . . . . . . . . . . . . . . . . . . . . . 9
4.2. Local Repair . . . . . . . . . . . . . . . . . . . . . . 10
4.3. Context Identifier . . . . . . . . . . . . . . . . . . . 13
4.3.1. Semantics . . . . . . . . . . . . . . . . . . . . . . 13
4.3.2. FEC . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.3.3. IGP Advertisement and Path Computation . . . . . . . 15
4.4. Protection Models . . . . . . . . . . . . . . . . . . . . 16
4.4.1. Co-located Protector . . . . . . . . . . . . . . . . 16
4.4.2. Centralized Protector . . . . . . . . . . . . . . . . 17
4.5. Transport Tunnel . . . . . . . . . . . . . . . . . . . . 19
4.6. Bypass Tunnel . . . . . . . . . . . . . . . . . . . . . . 20
4.7. Examples of Forwarding State . . . . . . . . . . . . . . 21
4.7.1. Co-located Protector Model . . . . . . . . . . . . . 21
4.7.2. Centralized Protector Model . . . . . . . . . . . . . 25
5. Restorative and Revertive Behaviors . . . . . . . . . . . . . 28
6. LDP Extensions . . . . . . . . . . . . . . . . . . . . . . . 29
6.1. Egress Protection Capability TLV . . . . . . . . . . . . 30
6.2. PW Label Distribution from Primary PE to Protector . . . 31
6.3. PW Label Distribution from Backup PE to Protector . . . . 31
6.4. Protection FEC Element TLV . . . . . . . . . . . . . . . 32
6.4.1. Encoding Format for PWid with IPv4 PE Addresses . . . 33
6.4.2. Encoding Format for Generalized PWid with IPv4 PE
Addresses . . . . . . . . . . . . . . . . . . . . . . 34
6.4.3. Encoding Format for PWid with IPv6 PE Addresses . . . 35
6.4.4. Encoding Format for Generalized PWid with IPv6 PE
Addresses . . . . . . . . . . . . . . . . . . . . . . 36
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 38
8. Security Considerations . . . . . . . . . . . . . . . . . . . 38
9. Acknowledgements References . . . . . . . . . . . . . . . . . . . . . . 39
10. References . . . 38
9.1. Normative References . . . . . . . . . . . . . . . . . . 39
9.2. Informative References . . . . 39
10.1. Normative References . . . . . . . . . . . . . 40
Acknowledgements . . . . . 39
10.2. Informative References . . . . . . . . . . . . . . . . . 40 . . 41
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 41

1. Introduction

Per [RFC3985, RFC4447, RFC5659], [RFC3985], [RFC8077], and [RFC5659], a pseudowire (PW) or PW
segment can be thought of as a connection between a pair of
forwarders hosted by two PEs, carrying an emulated layer-2 Layer 2 service
over a packet switched network (PSN). In the single-segment PW
(SS-PW) case, a forwarder binds a PW to an attachment circuit (AC).
In the multi-segment PW (MS-PW) case, a forwarder on a terminating PE
(T-PE) binds a PW segment to an AC, while a forwarder on a switching
PE (S-PE) binds one PW segment to another PW segment. In each
direction between the PEs, PW packets are transported by a PSN
tunnel, which is also called a transport tunnel.

In order to protect the PW service against network failures, it is
necessary to protect every link and node along the entire data path.
For the traffic in a given direction, this include includes ingress AC,
ingress (T-)PE, intermediate routers of the transport tunnel, S-PEs,
egress (T-)PE, and egress AC. To minimize service disruption upon a
failure, it is also desirable that each of these components is
protected by a fast protection mechanism based on local repair. Such
mechanism
mechanisms generally involves involve a bypass path that is pre-computed and
pre-installed in the data plane on the router upstream adjacent to an
anticipated failure. This router is referred to as a "point of local
repair" (PLR). The bypass path has the property that it can guide
traffic around the failure, while remaining unaffected by the
topology changes resulting from the failure. When the failure
occurs, the PLR can invoke the bypass path to achieve fast
restoration for the service.

Today, fast protection against ingress AC failure and ingress (T-)PE
failure can be achieved by using a multi-homed multihomed CE and redundant ACs,
such as a multi-chassis link aggregation group (MC-LAG). Fast
protection against the failure of an intermediate router of the
transport tunnel can be achieved through RSVP fast-reroute fast reroute [RFC4090]
or IP/LDP
fast-reroute [RFC5714, RFC5286]. fast reroute [RFC5286] [RFC5714]. However, there is no
equivalent mechanism that can be used against an egress AC failure,
an egress (T-)PE failure, or an S-PE failure. For these failures,
service restoration has to rely on global repair or control plane control-plane
convergence. Global repair normally involves the ingress CE or the
ingress (T-)PE switching traffic to an alternative path, based on
remote failure detection via PW status notification, end-to-end OAM,
Operations, Administration, and Maintenance (OAM), and others. Control plane
Control-plane convergence relies on control protocols to react on the
topology changes due to a failure. Compared to local repair, these
mechanisms are relatively slow in reacting to a failure and restoring
traffic.

This document is intended to serve addresses the above need. It specifies a fast
protection mechanism based on local repair to protect PWs against the
following endpoint failures. failures:

a. Egress AC failure.

b. Egress PE failure: Link or node failure of an egress PE of an SS-
PW,
SS-PW or a T-PE of an MS-PW.

c. Switching PE (S-PE) failure: Link or node failure of an S-PE of an
MS-PW.

The mechanism is applicable to LDP signaled LDP-signaled PWs. It is relevant to
networks with redundant PWs and multi-homed multihomed CEs. It is designed on
the basis of MPLS upstream label assignment and context-specific
label switching [RFC5331]. Fast protection refers to its ability to
restore traffic in the order of tens of milliseconds. Compared with
global repair and control plane control-plane convergence, this mechanism can
provide faster service restoration. However, it is intended to
complement these mechanisms, rather than replacing them, as networks
rely on them to eventually move traffic to fully functional
alternative paths.

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. [RFC2119].

3. Reference Models for Egress Endpoint Failures

This document refers to the following topologies to describe egress
endpoint failures and protection procedures.

3.1. Single-Segment PW
|<-------------- PW1 --------------->|

- PE1 -------------- P1 ---------------- PE2 -
/ \
/ \
CE1 CE2
\ /
\ /
- PE3 -------------- P2 ---------------- PE4 -

|<-------------- PW2 --------------->|

Figure 1

In Figure 1, the IP/MPLS network consists of PE and P routers. It
provides a PW service between CE1 and CE2. Each CE is multi-homed multihomed via
two ACs to two PEs. This forms two divergent paths between the CEs.
The first path uses PW1 between PE1 and PE2, and the second path uses
PW2 between PE3 and PE4. The For clarity, the transport tunnels of the
PWs and other links between the routers are not shown in this figure for
clarity. figure.

In general, a CE may operate the ACs in two modes when sending
traffic to the remote CE, i.e. i.e., active-standby mode and active-active
mode.

o In the active-standby mode, the CE chooses one AC as the active AC
and the corresponding path as the active path, and it uses the
other AC as the standby AC and the corresponding path as the
standby path. The CE only sends traffic on the active AC as long
as the active path is operational. The CE will only send traffic
on the standby AC after it detects a failure of the active path.
Note that the CE may receive traffic on the active or standby AC,
depending on whether the remote CE chooses the same active path
for the traffic of the reverse direction. In this document, even
if both CEs choose the same active path, each CE should still
anticipate receiving traffic on a standby AC, because the traffic
may be redirected to the standby path by the fast protection
mechanism.

o In the active-active mode, the CE treats both ACs and their
corresponding paths as active, active and sends traffic on both ACs in a
load balance
load-balancing fashion. In the reverse direction, the CE may
receive traffic on both ACs.

The above modes assume the traffic to be data traffic traffic, which is not
bound to a specific AC. This does not include control protocol control-protocol
traffic between the CEs, when the CE-CE control protocol control-protocol sessions or
adjacencies established on the two ACs are considered as distinct, distinct
rather than having a primary and backup relationship. In general, a
dual-homed CE should not make any explicit or implicit assumptions
regarding the specific AC from which it receives packets from the
remote CE.

For either mode, when considering the traffic flowing in a given
direction over an active path, this document views the ACs, PEs PEs, and
PWs to serve as serving primary or backup roles. In particular, the ACs, PEs PEs,
and PW PWs along this active path have primary roles, while those along
the other path have backup roles. Note that in the active-active
mode, each AC, PE, and PW on an active path has a primary role, role and
also a backup role protecting the other path path, which is also active.

For Figure 1, the following roles are assumed for the traffic going
from CE1 to CE2 via PW1.

Primary ingress AC: CE1-PE1

Primary ingress PE: PE1

Primary PW: PW1

Primary egress PE: PE2

Primary egress AC: PE2-CE2

Backup ingress AC: CE1-PE3

Backup ingress PE: PE3

Backup PW: PW2

Backup egress PE: PE4

Backup egress AC: PE4-CE2

Based on this schema, this document describes egress endpoint
failures and the fast protection mechanism on the per-active-path and
per-direction basis. In this case, an egress AC failure refers to
the failure of the AC PE2-CE2, and an egress node failure refers to
the failure of PE2. The ultimate goal is that when a failure occurs,
the traffic should be locally repaired, so that it can eventually
reach CE2 via the backup egress PE (PE4) and the backup egress AC
(PE4-CE2).

Subsequent to the local repair, either the current active path should
heal after the control plane converges on the new topology, topology or the
ingress CE should switch traffic from the primary path to the backup
path, depending on the failure scenario. In the latter case, the
ingress CE may perform the path switchover triggered by end-to-end
OAM (in-band or out-band), PW status notification, CE-PE control
protocols (e.g. LACP), (e.g., the Link Aggregation Control Protocol (LACP)), and
others. In the active-standby mode, this will promote the standby
path to a new active path. In the active-
active active-active mode, it will make
the other active path carry all the traffic between the two CEs. In
any case, this phase of restoration falls into the control plane control-plane
convergence and global repair category, and
hence category; hence, it is out of the scope
of this document. The purpose of the fast protection mechanism in
this document is to reduce traffic loss before this phase of
restoration takes place.

Note that in Figure 1, if the traffic in the reverse direction (i.e. (i.e.,
from CE2 to CE1) traverses the AC CE2-PE2 and PE2 as an active path,
the failure of PE2 and the failure of the AC PE2-CE2 will be
considered as ingress failures of the traffic. If CE2 can detect the
failures, it may protect the traffic by switching it to the backup
path via the AC CE2-PE4 and PE4. However, this is categorized as
ingress endpoint failure protection, and hence protection; hence, it is not handled by the
mechanism described in this document.

Figure 2 shows another possible scenario, where CE1 is single-homed
to PE1, while CE2 remains multi-homed multihomed to PE2 and PE4. From the
perspective of egress endpoint protection for the traffic going from
CE1 to CE2 over PW1, this scenario is the same as the scenario shown
in Figure 1.

|<-------------- PW1 --------------->|

------------- P1 ---------------- PE2 -
/ \
/ \
CE1 -- PE1 CE2
\ /
\ /
------------- P2 ---------------- PE4 -

|<-------------- PW2 --------------->|

Figure 2

For clarity, primary egress AC, primary egress PE, backup egress AC,
and backup egress PE may simply be referred to as primary AC, primary
PE, backup AC, and backup PE, respectively, when the context of a
discussion is egress endpoint.

3.2. Multi-Segment PW

|<--------------- PW1 --------------->|
|<----- SEG1 ----->|<----- SEG2 ----->|

- TPE1 -------------- SPE1 --------------- TPE2 -
/ \
/ \
CE1 CE2
\ /
\ /
- TPE3 -------------- SPE2 --------------- TPE4 -

|<----- SEG3 ----->|<----- SEG4 ----->|
|<--------------- PW2 --------------->|

Figure 3

Figure 3 shows a topology that is similar to Figure 1 but in an MS-PW
environment. PW1 and PW2 are both MS-PWs. PW1 is established
between TPE1 and TPE2, TPE2 and switched between segments SEG1 and SEG2 at
SPE1. PW2 is established between TPE3 and TPE4, TPE4 and switched between
segments SEG3 and SEG4 at SPE2. CE1 is multi-homed multihomed to TPE1 and TPE3.
CE2 is multi-homed multihomed to TPE2 and TPE4. The For clarity, the transport
tunnels of the PW segments are not shown in this figure for clarity. figure.

In this document, the following primary and backup roles are assigned
for the traffic going from CE1 to CE2:

Primary ingress AC: CE1-TPE1

Primary ingress T-PE: TPE1

Primary PW: PW1

Primary S-PE: SPE1

Primary egress T-PE: TPE2

Primary egress AC: TPE2-CE2

Backup ingress AC: CE1-TPE3

Backup ingress T-PE: TPE3

Backup PW: PW2

Backup S-PE: SPE2
Backup egress T-PE: TPE4

Backup egress AC: TPE4-CE2

In this case, an egress AC failure refers to the failure of the AC
TPE2-CE2. An egress node failure refers to the failure of TPE2. An
S-PE failure refers to the failure of SPE1.

For consistency with the SS-PW scenario, primary T-PEs and a primary
S-PEs may simply be referred to as primary PEs in this document,
where specifics are not required. Similarly, backup T-PEs and backup
S-PEs may be referred to as backup PEs.

4. Theory of Operation

The fast protection mechanism in this document provides three types
of protection for PWs, corresponding to the three types of failures
described in Section 1. 1:

a. Egress AC protection

b. Egress (T-)PE node protection

c. S-PE node protection

4.1. Applicability

The mechanism is applicable to LDP signaled LDP-signaled PWs in an environment
where an egress CE is multi-homed multihomed to a primary PE and a backup PE and
there exists a backup PW, as described in Section 3. The procedure
for S-PE node protection is applicable when there exists a backup
S-PE on the backup PW.

The mechanism assumes IP/MPLS transport tunnels, tunnels and is applicable to
tunnels with single path and ECMPs (equal cost multiple paths). equal-cost multipaths (ECMPs). As an
example of ECMPs, imagine a tunnel carrying one or multiple PWs and
traversing a router with ECMPs to a primary PE. The ECMPs consist of
at least one direct link to the PE, PE and some multi-hop paths to the
PE. Due to the direct link, the router is considered as a
penultimate hop of the tunnel, tunnel and can perform local detection and
repair for an egress node failure. The router normally uses a
hashing algorithm to distribute PW packets over the ECMPs, on a per-
PW or per-flow basis. Upon a failure of the direct link, i.e. i.e.,
transit link failure, the router removes the link from the hashing
algorithm, which automatically redistributes the traffic of the link
to the other paths of the ECMPs, achieving local repair. This
scenario is not the focus of this document. Upon a failure of the
PE, i.e. i.e., egress node failure, the router SHOULD perform local repair
by rerouting the entire traffic of the ECMPs, as the failure will
affect every path. If the router does not have a fast or reliable
mechanism to detect the egress node failure, it is RECOMMENDED that
the router SHOULD treat the failure of the direct link as an egress
node failure.

The mechanism is applicable to both best-effort and traffic
engineering (TE) transport tunnels. For TE transport tunnels which that
require bandwidth protection, TE bypass tunnels with reserved
bandwidth MAY be used to avoid congestion for rerouted traffic.

It is also RECOMMENDED that the mechanism SHOULD be used in
conjunction with global repair and control plane control-plane convergence, in such
a manner that the mechanism temporarily repairs a failed path by
using a bypass tunnel, and global repair and control plane control-plane
convergence eventually move traffic to a fully functional alternative
path.

4.2. Local Repair

The fast protection ability of the mechanism comes from local repair
performed by routers upstream adjacent to failures. Each of these
routers is referred to as a "point of local repair" (PLR). PLR. A PLR MUST be able to detect a
failure by using a rapid mechanism, such as
physical layer physical-layer failure
detection, Bidirectional Forwarding Detection (BFD) [RFC5880],
Seamless BFD (S-BFD) [RFC7880], and others. In anticipation of the
failure, the PLR MUST also pre-establish a bypass tunnel to a "protector",
"protector" and pre-install a bypass route for the bypass tunnel in
the data plane. The protector is either a backup PE or a router which that
will forward traffic to a backup PE. The bypass tunnel MUST have the
property that it will not be affected by the topology changes due to
the failure. Specifically, it MUST NOT traverse the primary PE or
the penultimate link of the protected transport tunnel, tunnel or share any SRLG (shared
shared risk link groups) groups (SRLGs) with the penultimate link. Upon
detecting the failure, the PLR invokes the bypass route in the data plane,
plane and reroutes PW traffic to the protector through the bypass
tunnel. The protector in turn sends the traffic to the target CE.
This procedure is referred to as local repair.

Different routers may serve as PLR and protector in different
scenarios.

o In egress AC protection, the PLR is the primary PE, and the
protector is the backup PE (Figure 4).

|<-------------- PW1 --------------->|

- PE1 -------------- P1 ---------------- PE2 -
/ PLR \
/ | \
CE1 bypass| CE2
\ | /
\ | /
- PE3 -------------- P2 ---------------- PE4 -
protector

|<-------------- PW2 --------------->|

Figure 4

o In egress PE node protection, the PLR is the penultimate hop
router of the transport tunnel of the primary PW, and the
protector is the backup PE (Figure 5).

|<-------------- PW1 --------------->|

- PE1 -------------- P1 ------- P3 ----- PE2 -
/ PLR \ \
/ \ \
CE1 bypass\ CE2
\ \ /
\ \ /
- PE3 -------------- P2 ---------------- PE4 -
protector

|<-------------- PW2 --------------->|

Figure 5

o In S-PE node protection, the PLR is the penultimate hop router of
the transport tunnel of the primary PW segment, and the protector
is the backup S-PE (Figure 6).

|<--------------- PW1 --------------->|
|<----- SEG1 ----->|<----- SEG2 ----->|

- TPE1 ----- P1 ----- SPE1 -------------- TPE2 -
/ PLR \ \
/ \ \
CE1 bypass\ CE2
\ \ /
\ \ /
- TPE3 --------------- SPE2 -------------- TPE4 -
protector

|<----- SEG3 ----->|<----- SEG4 ----->|
|<--------------- PW2 --------------->|

Figure 6

In egress AC protection, a PLR realizes its role based on
configuration of a "context identifier" identifier", which is introduced in this
document (Section 4.3). The PLR establishes a bypass tunnel to the
protector in the same fashion as a normal PSN tunnel.

In egress PE and S-PE node protection, a PLR is a transit router on
the transport tunnel, and it normally does not have knowledge of the
PW(s) carried by the transport tunnel. In this document, the PLR
simply computes and establishes a node protection node-protection bypass tunnel in
the same fashion as the normal IP/MPLS node protection, except that
with the notion of the context identifier, the bypass tunnel will be
established from the PLR to the protector (Section 4.6). Conversely,
when the router is no longer a PLR for egress PE or S-PE node
protection due to a change in network topology or the transport
tunnel's path, the router should revert to the role of regular
transit router, including PLR for transit link and node protection.

In local repair, a PLR simply switches all the traffic received on
the transport tunnel to the bypass tunnel. This requires that the
protector given by the bypass tunnel MUST be intended for all the PWs
carried by the transport tunnel. This is achieved by the ingress PE
using a context identifier to associate a PW with the specific pair
of {primary PE, protector} and map the PW to a transport tunnel
destined for the same {primary PE, protector}. The ingress PE MAY
map multiple PWs to the transport tunnel, if they share the {primary
PE, protector} in common.

In local repair, the PLR keeps the PW label intact in packets. This
obviates the need for the PLR to maintain bypass routes on a per-PW
basis,
basis and allows bypass tunnel sharing between PWs. On the other
hand, this imposes a requirement on the protector that it MUST be
able to forward the packets based on a PW label that is assigned by
the primary PE, PE and ensure that the traffic MUST reach the target CE
via a backup path. From the protector's perspective, this PW label
is an upstream assigned label [RFC5331]. To achieve this, the
protector MUST learn the PW label from the primary PE prior to the
failure,
failure and install a proper forwarding state for the PW label in a
dedicated label space associated with the primary PE. During local
repair, the protector MUST perform PW label lookup in this label
space.

The previous examples have shown the scenarios where the protectors
are backup (T/S-)PEs. (T-/S-)PEs. It is also possible that a protector is a
dedicated router to serve such a role, separate from the backup (T/
S-)PE.
(T-/S-)PE. During local repair, the PLR still reroutes traffic to
the protector through a bypass tunnel. The protector then forwards
the traffic to the backup (T/S-)PE, (T-/S-)PE, which further forwards the
traffic to the target CE via a backup AC or a backup PW segment.
More detail
will be described is included in Section 4.4.

4.3. Context Identifier

A protector may protect multiple primary PEs. The protector MUST
maintain a separate label space for each primary PE. Likewise, the
PWs terminated on a primary PE may be protected by multiple
protectors, each for a subset of the PWs. In any case, a given PW
MUST be associated with one and only one pair of {primary PE,
protector}.

This document introduces the notion of "context identifier" a context identifier to
facilitate protection establishment. A context identifier is an
IPv4/v6 address assigned to each ordered pair of {primary PE,
protector}. The address MUST be globally unique, unique or unique in the
address space of the network where the primary PE and the protector
reside.

4.3.1. Semantics

The semantics of a context identifier is twofold. twofold:

o A context identifier identifies a primary PE and an associated
protector. It represents the primary PE as the PW destination on
a
per protector per-protector basis. A given primary PE may be protected by
multiple protectors, each for a subset of the PWs terminated on
the primary PE. A distinct context identifier MUST be assigned to
each {primary PE, protector} pair.

The ingress PE of a PW learns the context identifier of the PW's
{primary PE, protector} from the primary PE via the Interface_ID
TLV
[RFC3471, RFC3472] [RFC3471] [RFC3472] in the LDP Label Mapping message of the
PW. The ingress PE then sets up or resolves a transport tunnel
with the context identifier, rather than a private IP address of
the primary PE, as the destination. This destination not only
makes the transport tunnel reach the primary PE, PE but also conveys
the identity of the protector to the PLR, which MUST use the
context identifier as the destination for the bypass tunnel to the
protector. The ingress PE MUST map only the PWs terminated by the
exact primary PE and protected by the exact protector to the
transport tunnel.

o A context identifier indicates the primary PE's label space on the
protector. The protector may protect PWs for multiple primary
PEs. For each primary PE, it MUST maintain a separate label space
to store the PW labels assigned by that primary PE. It associates
a PW label with a label space via the context identifier of the
{primary PE, protector}, as below.

In addition to the normal LDP PW signaling, the primary PE MUST
have a targeted LDP session with the protector, protector and advertise PW
labels to the protector via LDP Label Mapping messages
(Section 6). The primary PE MUST attach the context identifier to
each message. Upon receiving the message, the protector MUST
install the advertised PW label in the label space identified by
the context identifier.

When a PLR sets up or resolves a bypass tunnel to the protector,
it MUST use the context identifier rather than a private IP
address of the protector as the destination. The protector MUST
use the bypass tunnel, either the MPLS tunnel label or the IP
tunnel destination address, as the pointer to the corresponding
label space. The protector MUST forward PW packets received on
the bypass tunnel based on label lookup in that label space.

4.3.2. FEC

In an MPLS network, a context identifier represents a FEC (Forwarding Forwarding
Equivalence Class) Class (FEC) for transport tunnels and bypass tunnels
destined for it. For examples, example, it may be encoded in an LDP Prefix FEC
Element,
Element or in the "tunnel end point endpoint address" of an RSVP Session
object. The FEC is associated with a unique forwarding state on PLRs
and the protector, which cannot be shared with other FECs. Some MPLS
protocols (e.g. (e.g., LDP) support FEC aggregation [RFC3031]. In this
case, FEC aggregation MUST NOT be applied to a context identifier's
FEC, and every router MUST assign a unique label to the FEC.

4.3.3. IGP Advertisement and Path Computation

Using a context identifier as the destination for both the transport
tunnel and bypass tunnel requires coordination between the primary PE
and the protector in IGP advertisement of the context identifier in
the routing domain and TE domain. The context identifier should be
advertised in such a way that all the routers on the tunnels MUST be
able to independently reach the following common view of paths. paths:

o The transport tunnel MUST have the primary PE as the path
endpoint.

o The bypass tunnel MUST have the protector as the path endpoint.
In egress PE and S-PE node protection, the path MUST avoid the
primary PE.

There are generally two categories of approaches to achieve the
above.
above:

o The first category does not require an ingress PE or a PLR to have
knowledge of the PW egress endpoint protection schema. It does
not require any IGP extension for context identifier
advertisement. A context identifier is advertised by the primary
PE and the protector as an address reachable via both routers.
The ingress PE and the PLR can compute paths by using a normal
method, such as Dijkstra, CSPF (constrained constrained shortest path first),
LFA [RFC5286] first (CSPF),
Loop-Free Alternate (LFA) [RFC5286], and MRT Maximally Redundant Tree
(MRT) [RFC7812]. One example is to advertise a context identifier
as a virtual proxy node connected to the primary PE and the
protector, with the link between the proxy node and the primary PE
having a more preferable IGP and TE metric than the link between
the proxy node and the protector. The transport tunnel will
follow the shortest path or a TE path to the primary
PE, PE and be
terminated by the primary PE. The PLR will no longer view itself
as a penultimate hop of the transport tunnel, but rather two hops
away from the proxy node, via the primary PE. Hence, a node node-
protection bypass tunnel will be available via the protector to
the proxy node, but it will actually be terminated by the
protector.

o The second category requires a PLR to have knowledge of the PW
egress endpoint protection schema. The primary PE advertises the
context identifier as a regular IP address, while the protector
advertises it by using an explicit "context identifier" object, identifier object",
which MUST be understood by the PLR. The "context identifier" context identifier
object requires an IGP extension. In both the routing domain and
the TE domain, the context identifier is only reachable via the
primary PE. This ensures that the transport tunnel is terminated
by the primary PE. The PLR views itself as the penultimate hop of
the transport tunnel, and based on the IGP "context identifier" context identifier
object, it establishes or resolves a bypass tunnel to the
advertiser (i.e. (i.e., the protector), while avoiding the primary PE.

The mechanism in this document intends to be flexible on the approach
used by a network, as long as it satisfies the above requirements for
the transport tunnel path and bypass tunnel path. In theory, the
network can use one approach for context ID X and another approach
for context ID Y. For a given context ID, all relevant routers,
including primary PE, protector, and PLR, must support and agree on
the chosen approach. The coordination between the routers can be
achieved by configuration.

4.4. Protection Models

There are two protection models based on the location of a protector. protector:
the co-located protector model and the centralized protector model.
A network MAY use either model or both.

4.4.1. Co-located Protector

In this model, the protector is a backup PE that is directly
connected to the target CE via a backup AC, or it is a backup S-PE on
a backup PW. That is, the protector is co-located with the backup
(S-)PE. Examples of this model have been are shown in Figure Figures 4, Figure 5 5, and Figure 6 in
Section 4.2.

In egress AC protection and egress PE node protection, when a
protector receives traffic from the PLR, it forwards the traffic to
the CE via the backup AC. This is shown in Figure 7, where PE2 is
the PLR for egress AC failure, P3 is the PLR for PE2 failure, and PE4
(backup PE) is the protector.

|<-------------- PW1 --------------->|

- PE1 -------------- P1 ------- P3 ----- PE2 ----
/ PLR \ PLR \
/ \ | \
CE1 bypass\ |bypass CE2
\ \ | /
\ \ | /
- PE3 -------------- P2 ---------------- PE4 ----
protector

|<-------------- PW2 --------------->|

Figure 7
In S-PE node protection, when a protector receives traffic from the
PLR, it forwards the traffic over the next segment of the backup PW.
The T-PE of the backup PW in turn forwards the traffic to the CE via
a backup AC. This is shown in Figure 8, where P1 is the PLR for SPE1
failure, and SPE2 (backup S-PE) is the protector for SPE1. SPE2
receives traffic from P1, swaps SEG1's label to SEG4's label, and
forwards the traffic over a transport tunnel to TPE4.

|<--------------- PW1 --------------->|
|<----- SEG1 ----->|<----- SEG2 ----->|

- TPE1 ----- P1 ----- SPE1 -------------- TPE2 -
/ PLR \ \
/ \ \
CE1 bypass\ CE2
\ \ /
\ \ /
- TPE3 --------------- SPE2 -------------- TPE4 -
protector

|<----- SEG3 ----->|<----- SEG4 ----->|
|<--------------- PW2 --------------->|

Figure 8

In the co-located protector model, the number of context identifiers
needed by a network is the number of distinct {primary PE, backup PE}
pairs. From the perspective of scalability, the model is suitable
for networks where the number of primary PEs and the average number
of backup PEs per primary PE are both relatively low.

4.4.2. Centralized Protector

In this model, the protector is a dedicated P router or PE router
that serves the role. In egress AC protection and egress PE node
protection, the protector may or may not be a backup PE directly
connected to the target CE. In S-PE node protection, the protector
may or may not be a backup S-PE on the backup PW.

In egress AC protection and egress PE node protection, if the
protector is not directly connected to the CE, it forwards the
traffic to a backup PE, which in turn forwards the traffic to the CE
via a backup AC. This is shown in Figure 9, where the protector
receives traffic from P3 (PLR for egress PE failure) or PE2 (PLR for
egress AC failure), swaps PW1's label to PW2's label, and forwards
the traffic via a transport tunnel to PE4 (backup PE). The protector
may be protecting other PWs and other primary PEs as well, which well; for
clarity, this is not shown in this figure for clarity. the figure.

|<------------- PW1 --------------->|

- PE1 ------------- P1 ------- P3 ----- PE2 --
/ PLR \ PLR \
/ \ / \
/ bypass\ /bypass \
/ \ / \
CE1 protector CE2
\ \ /
\ transport\ /
\ tunnel \ /
\ \ /
- PE3 ------------- P2 -----------------PE4 --

|<------------- PW2 --------------->|

Figure 9

In S-PE node protection, if the protector is not a backup S-PE, it
forwards the traffic to the backup S-PE, which in turn forwards the
traffic over the next segment of the backup PW. Finally, the T-PE of
the backup PW forwards the traffic to the CE via the backup AC. This
is shown in Figure 10, where the protector receives traffic from P1
(PLR), swaps SEG1's label to SEG3's label, and forwards the traffic
via a transport tunnel to SPE2 (backup S-PE). SPE2 in turn performs
MS-PW switching from SEG3's label to SEG4's label, label and forwards the
traffic over a transport tunnel to TPE4 (backup T-PE). The protector
may be protecting other PW segments and other primary S-PEs as well,
which well;
for clarity, this is not shown in this figure for clarity. the figure.

|<--------------- PW1 --------------->|
|<----- SEG1 ----->|<----- SEG2 ----->|

- TPE1 ----- P1 ----- SPE1 -------------- TPE2 -
/ PLR \ \
/ \ \
/ bypass\ \
/ \ \
CE1 protector CE2
\ \ /
\ transport\ /
\ tunnel \ /
\ \ /
- TPE3 --------------- SPE2 -------------- TPE4 -

|<----- SEG3 ----->|<----- SEG4 ----->|
|<--------------- PW2 --------------->|

Figure 10

The centralized protector model allows multiple primary PEs to share
one protector. Each primary PE may need only one protector.
Therefore, the number of context identifiers needed by a network may
be bound to the number of primary PEs.

4.5. Transport Tunnel

A PW is associated with a pair of {primary PE, protector}, which is
represented by a unique context identifier. The ingress PE of the PW
sets up or resolves a transport tunnel by using the context
identifier rather than a private IP address of the primary PE as the
destination. This not only ensures that the PW is transported to the
primary PE, PE but also facilitates bypass tunnel establishment at PLR,
because the context identifier contains the identity of the protector
as well. This is also the case for a multi-segment PW, where the
ingress PE and egress PE are T/S-PEs. T-/S-PEs.

An ingress PE learns the association between a PW and a context
identifier from the primary PE, which MUST advertise the context
identifier as a "third party "third-party next hop" via the IPv4/v6 Interface_ID
TLV [RFC3471, RFC3472] [RFC3471] [RFC3472] in the LDP Label Mapping message of the PW.

In an ECMP scenario, a transport tunnel may have multiple penultimate
hop routers. Each of them SHOULD act as a PLR independently. Also
in an ECMP scenario, a penultimate hop router may have ECMPs to the
primary PE. At least one path of the ECMPs must be a direct link to
the primary PE, qualifying the router as a penultimate hop. The
other paths of the ECMPs may be direct links or indirect paths to the
primary PE. In egress PE node protection and S-PE node protection,
when a node failure is detected, or a link failure is detected on a
direct link and treated as a node failure, the penultimate hop router
SHOULD act as a PLR and reroute the entire traffic of the ECMPs to
the protector.

4.6. Bypass Tunnel

A PLR may protect multiple PWs associated with one or multiple pairs
of {primary PE, protector}. The PLR MUST establish a bypass tunnel
to each protector for each context identifier associated with that
protector. The destination of the bypass tunnel MUST be the context
identifier (Section 4.3.1). Since the PLR is a transit router of the
transport tunnel, it SHOULD derive the context identifier from the
destination of the transport tunnel.

For examples, example, in Figure Figures 7 and Figure 9, a bypass tunnel is established from
PE2 (PLR for egress AC failure) to the protector, and another bypass
tunnel is established from P3 (PLR for egress node failure) to the
protector. In Figure Figures 8 and Figure 10, a bypass tunnel is established from
P1 (PLR for S-PE failure) to the protector.

In local repair, a PLR reroutes traffic to the protector through a
bypass tunnel, with the PW label intact in the packets. This
normally involves pushing a label to the label stack, if the bypass
tunnel is an MPLS tunnel, or pushing an IP header to the packets, if
the bypass tunnel is an IP tunnel. Upon receipt of the packets, the
protector forwards them based on the PW label. Specifically, the
protector uses the bypass tunnel as a context to determine the
primary PE's label space. If the bypass tunnel is an MPLS tunnel,
the protector should have assigned a non-reserved label to the bypass tunnel, and
hence
tunnel; hence, this label can serve as the context. This label is
also called a "context label", as it is actually bound to the context
identifier. If the bypass tunnel is an IP tunnel, the context
identifier should be the destination address of the IP header.

To be useful for local repair, a bypass tunnel MUST have the property
that it is not affected by any topology changes caused by the
failure. It MUST NOT traverse the primary PE or the penultimate link
of the transport tunnel, or share any SRLG with the penultimate link.
A bypass tunnel may be a TE tunnel with reserved bandwidth to avoid
traffic congestion for rerouted traffic. A bypass tunnel should
remain effective during local repair, until the traffic is moved to
an alternative path, i.e. i.e., either the same PW over a fully functional
transport tunnel, tunnel or another fully functional PW.

There is little or no benefit to protect a bypass tunnel. Therefore,
a bypass tunnel SHOULD NOT be protected against a transit link
failure, transit node failure, or egress node failure.

4.7. Examples of Forwarding State

This section provides some detailed examples of forwarding state on
the PLR, protector, and other relevant routers.

A protector learns PW labels from all the primary PEs that it
protects (Section 6.2), 6.2) and maintains the PW labels in separate label
spaces on a per primary PE per-primary-PE basis. In the control plane, each label
space is identified by the context identifier of the corresponding
{primary PE, protector}. In the forwarding plane, the label space is
indicated by the bypass tunnel(s) destined for the context
identifier.

4.7.1. Co-located Protector Model

In Figure 11, PE4 is a co-located protector that protects PW1 against
egress AC failure and egress node failure. It maintains a label
space for PE2, which is identified by the context identifier of {PE2,
PE4}. It learns PW1's label from PE2, PE2 and installs an a forwarding entry
for the label in that label space. The nexthop next hop of the forwarding
entry indicates a label pop with an outgoing interface pointing to
the backup AC PE4-CE2.

|<-------------- PW1 --------------->|

- PE1 -------------- P1 ------- P3 ----- PE2 ------
/ PLR \ PLR \
/ \ | \
/ \ | \
CE1 bypass P4 P5 bypass CE2
\ \ | /
\ \ | /
\ \ | /
- PE3 -------------- P2 ---------------- PE4 ------
protector

|<-------------- PW2 --------------->|

PW1's label assigned by PE2: 100
PW2's label assigned by PE4: 200
On P3: </t>
Incoming label of transport tunnel to PE2: 1000
Outgoing label of transport tunnel to PE2: implicit null
Outgoing label of bypass tunnel to PE4: 2000
On PE2:
Outgoing label of bypass tunnel to PE4: 3000
On PE4:
Context label (incoming label of bypass tunnels): 999

Forwarding state on P3:
label 1000 -- primary nexthop: next hop: pop, to PE2
backup nexthop: next hop: swap 2000, to P4

Forwarding state on PE2:
label 100 -- primary nexthop: next hop: pop, to CE2
backup nexthop: next hop: push 3000, to P5

Forwarding state on P4:
label 2000 -- nexthop: next hop: swap 999, to PE4

Forwarding state on P5:
label 3000 -- nexthop: next hop: swap 999, to PE4

Forwarding state on PE4:
label 200 -- nexthop: next hop: pop, to CE2
label 999 -- nexthop: next hop: label table of PE2's label space

Label table of PE2's label space on PE4:
label 100 -- nexthop: next hop: pop, to CE2

Figure 11
In Figure 12, SPE2 is a co-located protector that protects PW1
against S-PE failure. It maintains a label space for SPE1, which is
identified by the context identifier of {SPE1, SPE2}. It learns
SEG1's label from SPE1, SPE1 and installs a forwarding entry in the label
space. The nexthop next hop of the forwarding entry indicates a label swap
to SEG4's label and a label push with the label of a transport tunnel
to TPE4.

|<--------------- PW1 --------------->|
|<----- SEG1 ----->|<----- SEG2 ----->|

- TPE1 ----- P1 ----- SPE1 --- P3 ------- TPE2 -
/ PLR \ \
/ \ \
CE1 bypass P2 CE2
\ \ /
\ \ /
- TPE3 --------------- SPE2 --- P4 ------- TPE4 -
protector

|<----- SEG3 ----->|<----- SEG4 ----->|
|<--------------- PW2 --------------->|

SEG1's label assigned by SPE1: 100
SEG2's label assigned by TPE2: 200
SEG3's label assigned by SPE2: 300
SEG4's label assigned by TPE4: 400
On P1:
Incoming label of transport tunnel to SPE1: 1000
Outgoing label of transport tunnel to SPE1: implicit null
Outgoing label of bypass tunnel to SPE2: 2000
On SPE1:
Outgoing label of transport tunnel to TPE2: 3000
On SPE2:
Outgoing label of transport tunnel to TPE4: 4000
Context label (incoming label of bypass tunnel): 999

Forwarding state on P1:
label 1000 -- primary nexthop: next hop: pop, to SPE1
backup nexthop: next hop: swap 2000, to P2

Forwarding state on SPE1:
label 100 -- nexthop: next hop: swap 200, push 3000, to P3

Forwarding state on P2:
label 2000 -- nexthop: next hop: swap 999, to SPE2

Forwarding state on SPE2:
label 300 -- nexthop: next hop: swap 400, push 4000, to P4
label 999 -- nexthop: next hop: label table of SPE1's label space

Label table of SPE1's label space on SPE2:
label 100 -- nexthop: next hop: swap 400, push 4000, to P4

Figure 12

4.7.2. Centralized Protector Model

In the centralized protector model, for each primary PW of which the
protector is not a backup (S-)PE, the protector MUST also learn the
label of the backup PW from the backup (S-)PE (Section 6.3). This is
the backup (S-)PE that the protector will forward traffic to. The
protector MUST install a forwarding entry with a label swap from the
primary PW's label to the backup PW's label and a label push with the
label of a transport tunnel to the backup (S-)PE.

In Figure 13, the protector is a centralized protector that protects
PW1 against egress AC failure and egress node failure. It maintains
a label space for PE2, which is identified by the context identifier
of {PE2, protector}. It learns PW1's label from PE2, PE2 and PW2's label
from PE4. It installs a forwarding entry for PW1's label in the
label space. The nexthop next hop of the forwarding entry indicates a label
swap to PW2's label and a label push with the label of a transport
tunnel to PE4.

|<-------------- PW1 --------------->|

- PE1 ------------- P1 ------- P3 ------ PE2 ----
/ PLR \ PLR \
/ \ / \
/ bypass P5 P6 bypass \
/ \ / \
/ \/ \
CE1 protector CE2
\ \ /
\ transport \ /
\ tunnel P7 /
\ \ /
\ \ /
- PE3 ------------- P2 ----------------- PE4 ----

|<-------------- PW2 --------------->|

PW1's label assigned by PE2: 100
PW2's label assigned by PE4: 200
On P3:
Incoming label of transport tunnel to PE2: 1000
Outgoing label of transport tunnel to PE2: implicit null
Outgoing label of bypass tunnel to protector: 2000
On PE2:
Outgoing label of bypass tunnel to protector: 3000
On protector:
Context label (incoming label of bypass tunnels): 999
Outgoing label of transport tunnel to PE4: 4000
Forwarding state on P3:
label 1000 -- primary nexthop: next hop: pop, to PE2
backup nexthop: next hop: swap 2000, to P5

Forwarding state on PE2:
label 100 -- primary nexthop: next hop: pop, to CE2
backup nexthop: next hop: push 3000, to P6

Forwarding state on P5:
label 2000 -- nexthop: next hop: swap 999, to protector

Forwarding state on P6:
label 3000 -- nexthop: next hop: swap 999, to protector

Forwarding state on P7:
label 4000 -- nexthop: next hop: pop, to PE4

Forwarding state on PE4:
label 200 -- nexthop: next hop: pop, to CE2

Forwarding state on protector:
label 999 -- nexthop: next hop: label table of PE2's label space

Label table of PE2's label space on protector:
label 100 -- nexthop: next hop: swap 200, push 4000, to P7

Figure 13

In Figure 14, the protector is a centralized protector that protects
the PW segment SEG1 of PW1 against the node failure of SPE1. It
maintains a label space for SPE1, which is identified by the context
identifier of {SPE1, protector}. It learns SEG1's label from SPE1, SPE1
and learns SEG3's label from SPE2. It installs a forwarding entry for
SEG1's label in the label space. The nexthop next hop of the forwarding
entry indicates a label swap to SEG3's label and a label push with
the label of a transport tunnel to TPE4.

|<--------------- PW1 --------------->|
|<----- SEG1 ----->|<----- SEG2 ----->|

- TPE1 ----- P1 ----- SPE1 --- P2 -------- TPE2 -
/ PLR \ \
/ \ \
/ bypass P4 \
/ \ \
/ \ \
CE1 protector CE2
\ \ /
\ \ /
\ transport P5 /
\ tunnel \ /
\ \ /
- TPE3 -------------- SPE2 --- P3 -------- TPE4 -

|<----- SEG3 ----->|<----- SEG4 ----->|
|<--------------- PW2 --------------->|

SEG1's label assigned by SPE1: 100
SEG2's label assigned by TPE2: 200
SEG3's label assigned by SPE2: 300
SEG4's label assigned by TPE4: 400
On P1:
Incoming label of transport tunnel to SPE1: 1000
Outgoing label of transport tunnel to SPE1: implicit null
Outgoing label of bypass tunnel to protector: 2000
On SPE1:
Outgoing label of transport tunnel to TPE2: 3000
On SPE2:
Outgoing label of transport tunnel to TPE4: 4000
On protector:
Context label (incoming label of bypass tunnel): 999
Outgoing label of transport tunnel to SPE2: 5000

Forwarding state on P1:
label 1000 -- primary nexthop: next hop: pop, to SPE1
backup nexthop: next hop: swap 2000, to P4

Forwarding state on SPE1:
label 100 -- nexthop: next hop: swap 200, push 3000, to P2

Forwarding state on P4:
label 2000 -- nexthop: next hop: swap 999, to protector

Forwarding state on P5:
label 5000 -- nexthop: next hop: pop, to SPE2

Forwarding state on SPE2:
label 300 -- nexthop: next hop: swap 400, push 4000, to P3

Forwarding state on protector:
label 999 -- nexthop: next hop: label table of SPE1's label space

Label table of SPE1's label space on protector:
label 100 -- nexthop: next hop: swap 300, push 5000, to P5

Figure 14

5. Restorative and Revertive Behaviors

Subsequent to local repair, there are three strategies for a network
to restore traffic to a fully functional alternative path. path:

o Global repair

If the ingress CE is multi-homed multihomed (Figure 1), it MAY switch the
traffic to the backup AC AC, which is bound to the backup PW.
Alternatively, if the ingress PE hosts a backup PW (Figure 2), the
ingress PE MAY switch the traffic to the backup PW. These
procedures are referred to as global repair. Possible triggers of
global repair include PW status notification, Virtual Circuit
Connectivity Verification (VCCV) [RFC5085, RFC5885], [RFC5085] [RFC5885], BFD, end-to-
end
end-to-end OAM between CEs, and others.

o Control plane Control-plane convergence

In egress PE node protection and S-PE node protection, it is
possible that the failure is limited to the link between the PLR
and the primary PE, whereas the primary PE is still operational.
In this case, the PLR or an upstream router on the transport
tunnel MAY reroute the tunnel around the link via an alternative
path to the primary PE. Thus, the transport tunnel can heal and
continue to carry the PW to the primary PE. This procedure is
driven by control plane control-plane convergence on the new topology.

o Local reversion

The PLR MAY move traffic back to the primary PW, after the failure
is resolved. In egress AC protection, upon detecting that the
primary AC is restored, the PLR MAY start forwarding traffic over
the AC again. Likewise, in egress PE node protection and S-PE
node protection, upon detecting that the primary PE is restored,
the PLR MAY re-establish the transport tunnel to the primary PE, PE
and move the traffic from the bypass tunnel back to the transport
tunnel. These procedures are referred to as local reversion.

It is RECOMMENDED that the fast protection mechanism SHOULD be used
in conjunction with global repair. Particularly in the case of
egress PE and S-PE node failures, if the ingress PE or the protector
loses communication with the egress PE or S-PE for an extensive
period of time, the LDP session may go down. Consequently, the
ingress PE may bring down the primary PW completely, or the protector
may remove the forwarding entry of the primary PW label. In either
case, the service will be disrupted. In other words, although the
mechanism can temporarily repair traffic, control plane control-plane state may
eventually expire if the failure persists. Therefore, global repair
SHOULD take place in a timely manner to move traffic to a fully
functional alternative path.

Control plane

Control-plane convergence may automatically happen as control plane control-plane
protocols react to the new topology. However, it is only applicable
to the specific link failure scenario described above.

Local reversion is optional. In the circumstances where the failure
is caused by resource flapping, local reversion MAY be dampened to
limit potential disruption. Local reversion MAY be disabled
completely by configuration.

6. LDP Extensions

As described in previous sections, a targeted LDP session MUST be
established between each pair of primary PE and protector. The
primary PE sends a Label Mapping message over this session to
advertise primary PW labels to the protector. In the centralized
protector model, a targeted LDP session MUST also be established
between a backup (S-)PE and the protector. The backup PE sends a
Label Mapping message over this session to advertise backup PW labels
to the protector.

To support the procedures, this document defines a new "Protection
FEC Element" TLV. The Label Mapping messages of both the LDP
sessions above MUST carry this TLV to identify a primary PW.
Specifically, in the centralized protector model, the Protection FEC
Element TLV advertised by a backup (S-)PE MUST match the one
advertised by the primary PE, so that the protector can associate the
primary PW's label with the backup PW's label, label and perform a label
swap. The backup (S-)PE builds such a Protection FEC Element TLV
based on local configuration.

This document also defines a new "Egress Protection Capability" TLV
as a new type of Capability Parameter TLV [RFC5561], to allow a
protector to announce its capability of processing the above
Protection FEC Element TLV and performing context specific context-specific label
switching for PW labels.

The procedures in this section are only applicable, applicable if the protector
advertises the Egress Protection Capability TLV, the primary PE
supports the advertisement of the Protection FEC Element TLV, and in
the centralized protector model, the backup PE also supports the
advertisement of the Protection FEC Element TLV.

6.1. Egress Protection Capability TLV

A protector MUST advertise the Egress Protection Capability TLV in
its Initialization message and Capability message, message over the LDP
session with a primary PE. In the centralized protector model, the
protector MUST also advertise the TLV over the LDP session with a
backup PE. The TLV carries one or multiple context identifiers. To
the primary PE, the TLV MUST carry the context identifier of the
{primary PE, protector}. In the centralized protector model, the TLV
MUST carry multiple context identifiers to the backup PE multiple context identifiers, PE, one for
each {primary PE, protector} where the backup PE serves as a backup
for the primary PE. This TLV MUST NOT be advertised by the primary
PE or the backup PE to the protector.

The processing of the Egress Protection Capability TLV by a receiving
router MUST follow the procedures defined in [RFC5561]. In
particular, the router MUST advertise PW information to the protector
by using the Protection FEC Element TLV, only after it has received
the Egress Protection Capability TLV from the protector. It MUST
validate each context identifier included in the TLV, TLV and advertise
the information of only the PWs that are associated with the context
identifier. It MUST withdraw previously advertised Protection FEC
TLVs, when the protector has withdrawn a previously advertised
context identifier or the entire Egress Protection Capability TLV via
a Capability message.

The encoding of the Egress Protection Capability TLV is defined as
below. It conforms to the format of Capability Parameter TLV
specified in [RFC5561].

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|U|F| Egress Protection (TBD) | (0x0974)| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|S| Reserved | |
+-+-+-+-+-+-+-+-+ |
| |
~ Capability Data = context identifier(s) ~
| |
| +-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 15
The U-bit MUST be set to 1 1, so that a receiver MUST silently ignore
this TLV if unknown to it, it and continue processing the rest of the
message.

The F-bit MUST be set to 0 since this TLV is sent only in
Initialization and Capability messages, which are not forwarded.

The TLV Code Point code point is TBD. It needs to be assigned by IANA. 0x0974.

The S-bit indicates whether the sender is advertising (S=1) or
withdrawing (S=0) the capability.

The "Capability Data" Capability Data field is encoded with the context identifiers of
the {primary PE, protector} pairs for which the advertiser is the
protector.

6.2. PW Label Distribution from Primary PE to Protector

A primary PE MUST advertise a primary PW's label to a protector by
sending a Label Mapping message. The message includes a Protection
FEC Element TLV (see Section 6.4 for encoding), encoding) and an Upstream-
Assigned Label TLV [RFC6389] encoded with the PW's label. The
combination of the Protection FEC Element TLV and the PW label
represents the primary PE's forwarding state for the PW. The Label
Mapping message MUST also carry an IPv4/v6 Interface_ID TLV [RFC6389,
RFC3471] [RFC3471]
[RFC6389] encoded with the context identifier of the {primary PE,
protector}.

The protector that receives this Label Mapping message MUST install a
forwarding entry for the PW label in the label space identified by
the context identifier. The nexthop next hop of the forwarding entry MUST
ensure that packets to be are sent towards the target CE via a backup AC or
a backup (S-)PE, depending on the protection scenario. The protector
MUST silently discard a Label Mapping message if the included context
identifier is unknown to it.

6.3. PW Label Distribution from Backup PE to Protector

In the centralized protector model, a backup PE MUST advertise a
backup PW's label to the protector by sending a Label Mapping
message. The message includes a Protection FEC Element TLV and a
Generic Label TLV encoded with the backup PW's label. This
Protection FEC Element MUST be identical to the Protection FEC
Element TLV that the primary PE advertises to the protector
(Section 6.2). This is achieved through configuration on the backup
PE. The context identifier MUST NOT be encoded in an Interface_ID
TLV in this message.

The protector that receives this Label Mapping message MUST associate
the backup PW with the primary PW, based on the common Protection FEC
Element TLV. It MUST distinguish between the Label Mapping message
from the primary PE and the Label Mapping message from the backup PE
based on the respective presence and absence of a context identifier
in the Interface_ID TLV. It MUST install a forwarding entry for the
primary PW's label in the label space identified by the context
identifier. The nexthop next hop of the forwarding entry MUST indicate a
label swap to the backup PW's label, followed by a label push or IP
header push for a transport tunnel to the backup PE.

6.4. Protection FEC Element TLV

The Protection FEC Element TLV has type 0x83. Its format is defined
as
below:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type(0x83) | Reserved | Encoding Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
~ PW Information ~
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 16

- Encoding Type

Type of encoding format of the PW Information field. The
following types are defined, corresponding to the PWid FEC Element
and Generalized PWid FEC Element defined in [RFC4447]. [RFC8077].

1 - PWid FEC Element with IPv4 PE addresses (section 6.4.1) (Section 6.4.1).

2 - Generalized PWid FEC Element with IPv4 PE addresses
(section 6.4.2)
(Section 6.4.2).

3 - PWid FEC Element with IPv6 PE addresses (section 6.4.3) (Section 6.4.3).

4 - Generalized PWid FEC Element with IPv6 PE addresses
(section 6.4.4)
(Section 6.4.4).

- Length
Length of the PW Information field in octets.

- PW Information

Field of variable length that specifies a PW.

6.4.1. Encoding Format for PWid with IPv4 PE Addresses

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type(0x83) | Reserved | Enc Type(1) | Length(20) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ingress PE IPv4 Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Egress PE IPv4 Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PW ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C| PW Type | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 17

- Ingress PE IPv4 Address

IPv4 address of the ingress PE of PW.

- Egress PE IPv4 Address

IPv4 address of the egress PE of PW.

- Group ID

An arbitrary 32-bit value that represents a group of PWs and that
is used to create groups in the PW space.

- PW ID

A non-zero 32-bit connection ID that, together with the PW Type
field, identifies a particular PW.

- Control word bit (C)
A bit that flags the presence of a control word on this PW. If
C = 1, control word is present; If if C = 0, control word is not
present.

- PW Type

A 15-bit quantity that represents the type of PW.

6.4.2. Encoding Format for Generalized PWid with IPv4 PE Addresses

Figure 18

- Ingress PE IPv4 Address

IPv4 address of the ingress PE of PW.

- Egress PE IPv4 Address

IPv4 address of the egress PE of PW.

- Control word bit (C)
A bit that flags the presence of a control word on this PW. If
C = 1, control word is present; If if C = 0, control word is not
present.

- PW Type

A 15-bit quantity that represents the type of PW.

- AGI Type, Length, Value, AGI Value

Attachment Group Identifier of PW.

- SAII AII Type, Length, Value, SAII Value

Source Attachment Individual Identifier of PW.

- TAII AII Type, Length, Value, TAII Value

Target Attachment Individual Identifier of PW.

6.4.3. Encoding Format for PWid with IPv6 PE Addresses

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type(0x83) | Reserved | Enc Type(3) | Length(44) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Ingress PE IPv6 Address ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Egress PE IPv6 Address ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PW ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C| PW Type | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 19

- Ingress PE IPv6 Address

IPv6 address of the ingress PE of PW. PW; 16 octets.

- Egress PE IPv6 Address

IPv6 address of the egress PE of PW. PW; 16 octets.

- Group ID

An arbitrary 32-bit value that represents a group of PWs and that
is used to create groups in the PW space.

- PW ID

A non-zero 32-bit connection ID that, together with the PW Type
field, identifies a particular PW.

- Control word bit (C)

A bit that flags the presence of a control word on this PW. If
C = 1, control word is present; If if C = 0, control word is not
present.

- PW Type

A 15-bit quantity that represents the type of PW.

6.4.4. Encoding Format for Generalized PWid with IPv6 PE Addresses
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type(0x83) | Reserved | Enc Type(4) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Ingress PE IPv6 Address ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Egress PE IPv6 Address ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C| PW Type | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AGI Type | Length | AGI Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ AGI Value (contd.) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AII Type | Length | SAII Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ SAII Value (contd.) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AII Type | Length | TAII Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ TAII Value (contd.) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 20

- Ingress PE IPv6 Address

IPv6 address of the ingress PE of PW. PW; 16 octets.

- Egress PE IPv6 Address

IPv6 address of the egress PE of PW. PW; 16 octets.

- Control word bit (C)

A bit that flags the presence of a control word on this PW. If
C = 1, control word is present; If if C = 0, control word is not
present.

- PW Type

A 15-bit quantity that represents the type of PW.

- AGI Type, Length, Value, AGI Value
Attachment Group Identifier of PW.

- SAII AII Type, Length, Value, SAII Value

Source Attachment Individual Identifier of PW.

- TAII AII Type, Length, Value, TAII Value

Target Attachment Individual Identifier of PW.

7. IANA Considerations

This document defines a new "Egress Egress Protection Capability" Capability TLV in
Section 6. The document requests IANA to assign an LDP TLV code
point has assigned value 0x0974 for it in the TLV. "TLV Type
Name Space" registry.

This document defines a new Protection FEC Element TLV, and uses the
LDP Protection FEC Type Name Space TLV in Section 6.
IANA assigned value 0x83 for it. The LDP
Protection FEC it in the "Forwarding Equivalence Class
(FEC) Type Name Space for this value needs to reference this
document, and it is requested Space" registry per RFC 7358. IANA has updated the
registry to update this also reference to the RFC
number for this document.

Value Hex Name Label Advertisement Discipline
-------------------------------------------------------------------
131 0x83 Protection FEC Element DU

8. Security Considerations

In this document, PW traffic can be temporarily rerouted by a PLR to
a protector. In the centralized protector scenario, the traffic can
be further rerouted to a backup PE. In the control plane, there is a
targeted LDP session between a primary PE and a protector. In the
centralized protector scenario, there is also a targeted LDP session
between a backup PE and a protector. In all scenarios, traffic
rerouting via PLRs, protectors protectors, and backup PEs is planned and
anticipated, and backup PEs can be used anyway to host PWs and LDP
sessions. Hence, the rerouted traffic and the LDP sessions
introduced in this document should not be viewed as a new security
threat.

In general, [RFC5920] describes the security framework for MPLS
networks. [RFC3209] describes the security considerations for RSVP
LSPs.
Label Switched Paths (LSPs). [RFC5036] describes the security
considerations for the base LDP specification. [RFC5561] describes
the security considerations
which that apply when using the LDP capability
mechanism. All these security framework frameworks and considerations apply to
this document as well.

9. Acknowledgements

This document leverages work done by Hannes Gredler, Yakov Rekhter,
Minto Jeyananth, Kevin Wang and several on MPLS edge protection.
Thanks to Nischal Sheth and Bhupesh Kothari for their contribution.
Thanks to John E Drake, Andrew G Malis, Alexander Vainshtein, Stewart
Bryant, and Mach Chen for valuable comments that helped shape this
document and improve its clarity.

10. References

10.1.

9.1. Normative References

[RFC4447]

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.

[RFC8077] Martini, L., Ed., Rosen, E., El-Aawar, N., Smith, T., Ed. and G. Heron, Ed., "Pseudowire Setup and
Maintenance Using the Label Distribution Protocol (LDP)",
STD 84, RFC 4447, 8077, DOI 10.17487/RFC4447, April 2006,
<http://www.rfc-editor.org/info/rfc4447>. 10.17487/RFC8077, February 2017,
<http://www.rfc-editor.org/info/rfc8077>.

[RFC5331] Aggarwal, R., Rekhter, Y., and E. Rosen, "MPLS Upstream
Label Assignment and Context-Specific Label Space",
RFC 5331, DOI 10.17487/RFC5331, August 2008,
<http://www.rfc-editor.org/info/rfc5331>.

[RFC5561] Thomas, B., Raza, K., Aggarwal, S., Aggarwal, R., and JL.
Le Roux, "LDP Capabilities", RFC 5561,
DOI 10.17487/RFC5561, July 2009,
<http://www.rfc-editor.org/info/rfc5561>.

[RFC3471] Berger, L., Ed., "Generalized Multi-Protocol Label
Switching (GMPLS) Signaling Functional Description",
RFC 3471, DOI 10.17487/RFC3471, January 2003,
<http://www.rfc-editor.org/info/rfc3471>.

[RFC3472] Ashwood-Smith, P., Ed. and L. Berger, Ed., "Generalized
Multi-Protocol Label Switching (GMPLS) Signaling
Constraint-based Routed Label Distribution Protocol (CR-
LDP) Extensions", RFC 3472, DOI 10.17487/RFC3472, January
2003, <http://www.rfc-editor.org/info/rfc3472>.

[RFC6389] Aggarwal, R. and JL. Le Roux, "MPLS Upstream Label
Assignment for LDP", RFC 6389, DOI 10.17487/RFC6389,
November 2011, <http://www.rfc-editor.org/info/rfc6389>.

[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,
<http://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,
<http://www.rfc-editor.org/info/rfc5286>.

[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,
<http://www.rfc-editor.org/info/rfc7812>.

[RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
Label Switching Architecture", RFC 3031,
DOI 10.17487/RFC3031, January 2001,
<http://www.rfc-editor.org/info/rfc3031>.

10.2.

9.2. Informative References

[RFC5920] Fang, L., Ed., "Security Framework for MPLS and GMPLS
Networks", RFC 5920, DOI 10.17487/RFC5920, July 2010,
<http://www.rfc-editor.org/info/rfc5920>.

[RFC3985] Bryant, S., Ed. and P. Pate, Ed., "Pseudo Wire Emulation
Edge-to-Edge (PWE3) Architecture", RFC 3985,
DOI 10.17487/RFC3985, March 2005,
<http://www.rfc-editor.org/info/rfc3985>.

[RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,
<http://www.rfc-editor.org/info/rfc3209>.

[RFC5036] Andersson, L., Ed., Minei, I., Ed., and B. Thomas, Ed.,
"LDP Specification", RFC 5036, DOI 10.17487/RFC5036,
October 2007, <http://www.rfc-editor.org/info/rfc5036>.

[RFC5659] Bocci, M. and S. Bryant, "An Architecture for Multi-
Segment Pseudowire Emulation Edge-to-Edge", RFC 5659,
DOI 10.17487/RFC5659, October 2009,
<http://www.rfc-editor.org/info/rfc5659>.

[RFC5714] Shand, M. and S. Bryant, "IP Fast Reroute Framework",
RFC 5714, DOI 10.17487/RFC5714, January 2010,
<http://www.rfc-editor.org/info/rfc5714>.

[RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
<http://www.rfc-editor.org/info/rfc5880>.

[RFC5085] Nadeau, T., Ed. and C. Pignataro, Ed., "Pseudowire Virtual
Circuit Connectivity Verification (VCCV): A Control
Channel for Pseudowires", RFC 5085, DOI 10.17487/RFC5085,
December 2007, <http://www.rfc-editor.org/info/rfc5085>.

[RFC5885] Nadeau, T., Ed. and C. Pignataro, Ed., "Bidirectional
Forwarding Detection (BFD) for the Pseudowire Virtual
Circuit Connectivity Verification (VCCV)", RFC 5885,
DOI 10.17487/RFC5885, June 2010,
<http://www.rfc-editor.org/info/rfc5885>.

[RFC7880] Pignataro, C., Ward, D., Akiya, N., Bhatia, M., and S.
Pallagatti, "Seamless Bidirectional Forwarding Detection
(S-BFD)", RFC 7880, DOI 10.17487/RFC7880, July 2016,
<http://www.rfc-editor.org/info/rfc7880>.

Acknowledgements

This document leverages work done by Hannes Gredler, Yakov Rekhter,
Minto Jeyananth, Kevin Wang, and several others on MPLS edge
protection. Thanks to Nischal Sheth and Bhupesh Kothari for their
contributions. Thanks to John E. Drake, Andrew G. Malis, Alexander
Vainshtein, Stewart Bryant, and Mach(Guoyi) Chen for valuable
comments that helped shape this document and improve its clarity.

Authors' Addresses

Yimin Shen
Juniper Networks
10 Technology Park Drive
Westford, MA 01886
USA
United States of America

Phone: +1 9785890722
Email: yshen@juniper.net

Rahul Aggarwal
Arktan, Inc Inc.

Email: raggarwa_1@yahoo.com

Wim Henderickx
Nokia
Copernicuslaan 50
2018 Antwerp
Belgium

Email: wim.henderickx@nokia.com
Yuanlong Jiang
Huawei Technologies Co., Ltd.
Bantian, Longgang district
Shenzhen 518129
China

Email: jiangyuanlong@huawei.com