Internet Engineering Task Force (IETF) W. Cheng
Request for Comments: 8185 L. Wang
Category: Standards Track H. Li
ISSN: 2070-1721 China Mobile
J. Dong
Huawei Technologies
A. D'Alessandro
Telecom Italia
June 2017
Dual-Homing Coordination
for MPLS Transport Profile (MPLS-TP) Pseudowires Protection
Abstract
In some scenarios, MPLS Transport Profile (MPLS-TP) pseudowires (PWs)
(RFC 5921) may be statically configured when a dynamic control plane
is not available. A fast protection mechanism for MPLS-TP PWs is
needed to protect against the failure of an Attachment Circuit (AC),
the failure of a Provider Edge (PE), or a failure in the Packet
Switched Network (PSN). The framework and typical scenarios of dual-
homing PW local protection are described in RFC 8184. This document
proposes a dual-homing coordination mechanism for MPLS-TP PWs that is
used for state exchange and switchover coordination between the dual-
homing PEs for dual-homing PW local protection.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc8185.
Cheng, et al. Standards Track [Page 1]RFC 8185 Dual-Homing Coordination for MPLS-TP PWs June 2017
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. Overview of the Proposed Solution . . . . . . . . . . . . . . 4
4. Protocol Extensions for Dual-Homing MPLS-TP PW Protection . . 5
4.1. Information Exchange Between Dual-Homing PEs . . . . . . 5
4.2. Protection Procedures . . . . . . . . . . . . . . . . . . 9
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
6. Security Considerations . . . . . . . . . . . . . . . . . . . 13
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
7.1. Normative References . . . . . . . . . . . . . . . . . . 14
7.2. Informative References . . . . . . . . . . . . . . . . . 15
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17
Cheng, et al. Standards Track [Page 2]RFC 8185 Dual-Homing Coordination for MPLS-TP PWs June 2017
1. Introduction
[RFC6372], [RFC6378], and [RFC7771] describe the framework and
mechanism of MPLS Transport Profile (MPLS-TP) linear protection,
which can provide protection for the MPLS Label Switched Path (LSP)
and pseudowires (PWs) between the edge nodes. These mechanisms
cannot protect against the failure of the Attachment Circuit (AC) or
the edge nodes. [RFC6718] and [RFC6870] specify the PW redundancy
framework and mechanism for protecting the AC or edge node against
failure by adding one or more edge nodes, but it requires PW
switchover in case of an AC failure; also, PW redundancy relies on
Packet Switched Network (PSN) protection mechanisms to protect
against the failure of PW.
In some scenarios such as mobile backhauling, the MPLS PWs are
provisioned with dual-homing topology in which at least the Customer
Edge (CE) node on one side is dual-homed to two Provider Edge (PE)
nodes. If a failure occurs in the primary AC, operators usually
prefer to perform local switchover in the dual-homing PE side and
keep the working pseudowire unchanged, if possible. This is to avoid
massive PW switchover in the mobile backhaul network due to AC
failure in the mobile core site; such massive PW switchover may in
turn lead to congestion caused by migrating traffic away from the
preferred paths of network planners. Similarly, as multiple PWs
share the physical AC in the mobile core site, it is preferable to
keep using the working AC when one working PW fails in the PSN to
potentially avoid unnecessary switchover for other PWs. To meet the
above requirements, a fast dual-homing PW protection mechanism is
needed to protect against failure in the AC, the PE node, and the
PSN.
[RFC8184] describes a framework and several scenarios of dual-homing
PW local protection. This document proposes a dual-homing
coordination mechanism for static MPLS-TP PWs; the mechanism is used
for information exchange and switchover coordination between the
dual-homing PEs for the dual-homing PW local protection. The
proposed mechanism has been implemented and deployed in several
mobile backhaul networks that use static MPLS-TP PWs for the
backhauling of mobile traffic from the radio access sites to the core
site.
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
Cheng, et al. Standards Track [Page 3]
RFC 8185 Dual-Homing Coordination for MPLS-TP PWs June 2017
3. Overview of the Proposed Solution
Linear protection mechanisms for the MPLS-TP network are defined in
[RFC6378], [RFC7271], and [RFC7324]. When such mechanisms are
applied to PW linear protection [RFC7771], both the working PW and
the protection PW are terminated on the same PE node. In order to
provide dual-homing protection for MPLS-TP PWs, some additional
mechanisms are needed.
In MPLS-TP PW dual-homing protection, the linear protection mechanism
(as defined in [RFC6378], [RFC7271], and [RFC7324]) on the single-
homing PE (e.g., PE3 in Figure 1) is not changed, while on the dual-
homing side, the working PW and protection PW are terminated on two
dual-homing PEs (e.g., PE1 and PE2 in Figure 1), respectively, to
protect against a failure occurring in a PE or a connected AC. As
described in [RFC8184], a dedicated Dual-Node Interconnection (DNI)
PW is used between the two dual-homing PE nodes to forward the
traffic. In order to utilize the linear protection mechanism
[RFC7771] in the dual-homing PEs scenario, coordination between the
dual-homing PE nodes is needed so that the dual-homing PEs can switch
the connection between the AC, the service PW, and the DNI-PW
properly in a coordinated fashion by the forwarder.
+----------------------------------+
| PE1 |
+----------------------------------+ +----+
| | | Working | |
X Forwarder + Service X-------------X |
/| | PW | Service PW1 | |
AC1 / +--------+--------+ | | |
/ | DNI-PW | | | |
+---* +--------X--------+----------------+ | | +---+
| | ^ | | | |
|CE1| | DNI-PW |PE3 +---|CE2|
| | | | | | |
| | V | | | |
+---* +--------X--------+----------------+ | | +---+
\ | DNI-PW | | | |
AC2 \ +--------+--------+ | Protection | |
\| | Service X-------------X |
X Forwarder + PW | Service PW2 | |
| | | +----+
+----------------------------------+
| PE2 |
+----------------------------------+
Figure 1: Dual-Homing Protection with DNI-PW
Cheng, et al. Standards Track [Page 4]RFC 8185 Dual-Homing Coordination for MPLS-TP PWs June 2017
4. Protocol Extensions for Dual-Homing MPLS-TP PW Protection
In dual-homing MPLS-TP PW local protection, the forwarding states of
the dual-homing PEs are determined by the forwarding state machine in
Table 1.
+-----------+---------+--------+---------------------+
|Service PW | AC | DNI-PW | Forwarding Behavior |
+-----------+---------+--------+---------------------+
| Active | Active | Up |Service PW <-> AC |
+-----------+---------+--------+---------------------+
| Active | Standby | Up |Service PW <-> DNI-PW|
+-----------+---------+--------+---------------------+
| Standby | Active | Up | DNI-PW <-> AC |
+-----------+---------+--------+---------------------+
| Standby | Standby | Up | Drop all packets |
+-----------+---------+--------+---------------------+
| Active | Active | Down |Service PW <-> AC |
+-----------+---------+--------+---------------------+
| Active | Standby | Down | Drop all packets |
+-----------+---------+--------+---------------------+
| Standby | Active | Down | Drop all packets |
+-----------+---------+--------+---------------------+
| Standby | Standby | Down | Drop all packets |
+-----------+---------+--------+---------------------+
Table 1: Dual-Homing PE Forwarding State Machine
In order to achieve dual-homing MPLS-TP PW protection, coordination
between the dual-homing PE nodes is needed to exchange the PW status
and protection coordination requests.
4.1. Information Exchange Between Dual-Homing PEs
The coordination information will be sent on the DNI-PW over the
Generic Associated Channel (G-ACh) as described in [RFC5586]. A new
G-ACh channel type is defined for the dual-homing coordination
between the dual-homing PEs of MPLS-TP PWs. This channel type can be
used for the exchange of different types of information between the
dual-homing PEs. This document uses this channel type for the
exchange of PW status and switchover coordination between the dual-
homing PEs. Other potential usages of this channel type are for
further study and are out of the scope of this document.
Cheng, et al. Standards Track [Page 5]RFC 8185 Dual-Homing Coordination for MPLS-TP PWs June 2017
The MPLS-TP Dual-Homing Coordination (DHC) message is sent on the
DNI-PW between the dual-homing PEs. The format of the MPLS-TP DHC
message is shown 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 1|Version| Reserved | DHC Channel Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Dual-Homing PEs Group ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TLV Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ TLVs ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: MPLS-TP Dual-Homing Coordination Message
The first 4 octets is the common G-ACh header as specified in
[RFC5586]. The DHC Channel Type is the G-ACh channel type code point
assigned by IANA (0x0009).
The Dual-Homing Group ID is a 4-octet unsigned integer to identify
the dual-homing group to which the dual-homing PEs belong. It MUST
be the same at both PEs in the same group.
The TLV Length field specifies the total length in octets of the
subsequent TLVs.
In this document, two TLVs are defined in the MPLS-TP Dual-Homing
Coordination message for dual-homing MPLS-TP PW protection:
Type Description Length
1 PW Status 20 bytes
2 Dual-Node Switching 16 bytes
The PW Status TLV is used by a dual-homing PE to report its service
PW status to the other dual-homing PE in the same dual-homing group.
Cheng, et al. Standards Track [Page 6]RFC 8185 Dual-Homing Coordination for MPLS-TP PWs June 2017
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=1 (PW Status) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination Dual-Homing PE Node_ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Dual-Homing PE Node_ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DNI-PW ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags |P|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service PW Status |D|F|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: PW Status TLV
The Length field specifies the length in octets of the value field of
the TLV.
The Destination Dual-Homing PE Node_ID is the 32-bit identifier of
the receiver PE [RFC6370], which supports both IPv4 and IPv6
environments. Usually it is the same as the Label Switching Router
ID (LSR ID) of the receiver PE.
The Source Dual-Homing PE Node_ID is the 32-bit identifier of the
sending PE [RFC6370], which supports both IPv4 and IPv6 environments.
Usually it is the same as the LSR ID of the sending PE.
The DNI-PW ID field contains the 32-bit PW ID [RFC8077] of the DNI-
PW.
The Flags field contains 32-bit flags, in which:
o The P (Protection) bit indicates whether the Source Dual-Homing PE
is the working PE (P=0) or the protection PE (P=1).
o Other bits are reserved for future use, which MUST be set to 0 on
transmission and MUST be ignored upon receipt.
The Service PW Status field indicates the status of the service PW
between the sending PE and the remote PE. Currently, two bits are
defined in the Service PW Status field:
o F bit: If set, it indicates Signal Fail (SF) [RFC6378] on the
service PW. It can be either a local request generated by the PE
itself or a remote request received from the remote PE.
Cheng, et al. Standards Track [Page 7]RFC 8185 Dual-Homing Coordination for MPLS-TP PWs June 2017
o D bit: If set, it indicates Signal Degrade (SD) [RFC6378] on the
service PW. It can be either a local request or a remote request
received from the remote PE.
o Other bits are reserved for future use, which MUST be set to 0 on
transmission and MUST be ignored upon receipt.
The Dual-Node Switching TLV is used by one dual-homing PE to send
protection state coordination to the other PE in the same dual-homing
group.
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=2 (Dual-Node Switching) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination Dual-Homing PE Node_ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Dual-Homing PE Node_ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DNI-PW ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags |S|P|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Dual-Node Switching TLV
The Length field specifies the length in octets of the value field of
the TLV.
The Destination Dual-Homing PE Node_ID is the 32-bit identifier of
the receiver PE [RFC6370]. Usually it is the same as the LSR ID of
the receiver PE.
The Source Dual-Homing PE Node_ID is the 32-bit identifier of the
sending PE [RFC6370]. Usually it is the same as the LSR ID of the
sending PE.
The DNI-PW ID field contains the 32-bit PW-ID [RFC8077] of the DNI-
PW.
The Flags field contains 32-bit flags, in which:
o The P (Protection) bit indicates whether the Source Dual-Homing PE
is the working PE (P=0) or the protection PE (P=1).
Cheng, et al. Standards Track [Page 8]RFC 8185 Dual-Homing Coordination for MPLS-TP PWs June 2017
o The S (PW Switching) bit indicates which service PW is used for
forwarding traffic. It is set to 0 when traffic will be
transported on the working PW, and it is set to 1 if traffic will
be transported on the protection PW. The value of the S bit is
determined by the protection coordination mechanism between the
dual-homing PEs and the remote PE.
o Other bits are reserved for future use, which MUST be set to 0 on
transmission and MUST be ignored upon receipt.
When a change of service PW status is detected by one of the dual-
homing PEs, it MUST be reflected in the PW Status TLV and sent to the
other dual-homing PE as quickly as possible to allow for fast
protection switching using three consecutive DHC messages. This set
of three messages allows for fast protection switching even if one or
two of these packets are lost or corrupted. After the transmission
of the three rapid messages, the dual-homing PE MUST send the most
recently transmitted service PW status periodically to the other
dual-homing PE on a continual basis using the DHC message.
When one dual-homing PE determines that the active service PW needs
to be switched from the working PW to the protection PW, it MUST send
the Dual-Node Switching TLV to the other dual-homing PE as quickly as
possible to allow for fast protection switching using three
consecutive DHC messages. After the transmission of the three
messages, the protection PW would become the active service PW, and
the dual-homing PE MUST send the most recently transmitted Dual-Node
Switching TLV periodically to the other dual-homing PE on a continual
basis using the DHC message.
It is RECOMMENDED that the default interval of the first three rapid
DHC messages be 3.3 ms, similar to [RFC6378], and the default
interval of the subsequent messages is 1 second. Both the default
interval of the three consecutive messages as well as the default
interval of the periodic messages SHALL be configurable by the
operator.
4.2. Protection Procedures
The dual-homing MPLS-TP PW protection mechanism can be deployed with
the existing AC redundancy mechanisms. On the PSN side, a PSN tunnel
protection mechanism is not required, as the dual-homing PW
protection can also protect if a failure occurs in the PSN.
This section uses the one-side dual-homing scenario as an example to
describe the dual-homing PW protection procedures; the procedures for
a two-side dual-homing scenario would be similar.
Cheng, et al. Standards Track [Page 9]RFC 8185 Dual-Homing Coordination for MPLS-TP PWs June 2017
On the dual-homing PE side, the role of working and protection PE are
set by the management system or local configuration. The service PW
connecting to the working PE is the working PW, and the service PW
connecting to the protection PE is called the protection PW.
On the single-homing PE side, it treats the working PW and protection
PW as if they terminate on the same remote PE node, thus normal MPLS-
TP protection coordination procedures still apply on the single-
homing PE.
The forwarding behavior of the dual-homing PEs is determined by the
components shown in the figure below:
+---------------------------------+ +-----+
| PE1 (Working PE) | | |
+---------------------------------+ PW1 | |
| | | Working | |
+ Forwarder + Service X<-------->X |
/| | PW | | |
/ +--------+--------+ | | |
AC1 / | DNI-PW | | | |
/ +--------X--------+---------------+ | |
+-----+/ AC ^ DNI-PW | | +---+
| CE1 |redundancy | | PE3 +--|CE2|
+-----+ mechanism | DHC message | | +---+
\ V exchange | |
AC2 \ +--------X--------+---------------+ | |
\ | DNI-PW | | | |
\ +--------+--------+ | PW2 | |
\| | Service |Protection| |
+ Forwarder + PW X<-------->X |
| | | PSC | |
+---------------------------------+ message | |
| PE2 (Protection PE) | exchange | |
+---------------------------------+ +-----+
Figure 5: Components of One-Side Dual-Homing PW Protection
In Figure 5, for each dual-homing PE, the service PW is the PW used
to carry service between the dual-homing PE and the remote PE. The
state of the service PW is determined by the Operation,
Administration, and Maintenance (OAM) mechanisms between the dual-
homing PEs and the remote PE.
The DNI-PW is provisioned between the two dual-homing PE nodes. It
is used to bridge traffic when a failure occurs in the PSN or in the
ACs. The state of the DNI-PW is determined by the OAM mechanism
between the dual-homing PEs. Since the DNI-PW is used to carry both
Cheng, et al. Standards Track [Page 10]RFC 8185 Dual-Homing Coordination for MPLS-TP PWs June 2017
the DHC messages and the service traffic during protection switching,
it is important to ensure the robustness of the DNI-PW. In order to
avoid the DNI-PW failure due to the failure of a particular link, it
is RECOMMENDED that multiple diverse links be deployed between the
dual-homing PEs and the underlying Label Switched Path (LSP)
protection mechanism SHOULD be enabled.
The AC is the link that connects a dual-homing PE to the dual-homed
CE. The status of AC is determined by the existing AC redundancy
mechanisms; this is out of the scope of this document.
In order to perform dual-homing PW local protection, the service PW
status and Dual-Node Switching coordination requests are exchanged
between the dual-homing PEs using the DHC message defined in
Section 4.1.
Whenever a change of service PW status is detected by a dual-homing
PE, it MUST be reflected in the PW Status TLV and sent to the other
dual-homing PE immediately using the three consecutive DHC messages.
After the transmission of the three rapid messages, the dual-homing
PE MUST send the most recently transmitted service PW status
periodically to the other dual-homing PE on a continual basis using
the DHC message. This way, both dual-homing PEs have the status of
the working and protection PW consistently.
When there is a switchover request either generated locally or
received on the protection PW from the remote PE, based on the status
of the working and protection service PW along with the local and
remote request of the protection coordination between the dual-homing
PEs and the remote PE, the active/standby state of the service PW can
be determined by the dual-homing PEs. As the remote protection
coordination request is transmitted over the protection path, in this
case the active/standby status of the service PW is determined by the
protection PE in the dual-homing group.
If it is determined on one dual-homing PE that switchover of the
service PW is needed, this dual-homing PE MUST set the S bit in the
Dual-Node Switching TLV and send it to the other dual-homing PE
immediately using the three consecutive DHC messages. With the
exchange of service PW status and the switching request, both dual-
homing PEs are consistent on the active/standby forwarding status of
the working and protection service PWs. The status of the DNI-PW is
determined by PW OAM mechanism as defined in [RFC5085], and the
status of ACs is determined by existing AC redundancy mechanisms:
both are out of the scope of this document. The forwarding behavior
on the dual-homing PE nodes is determined by the forwarding state
machine as shown in Table 1.
Cheng, et al. Standards Track [Page 11]
RFC 8185 Dual-Homing Coordination for MPLS-TP PWs June 2017
Using the topology in Figure 5 as an example, in normal state, the
working PW (PW1) is in active state, the protection PW (PW2) is in
standby state, the DNI-PW is up, and AC1 is in active state according
to the AC redundancy mechanism. According to the forwarding state
machine in Table 1, traffic will be forwarded through the working PW
(PW1) and the primary AC (AC1). No traffic will go through the
protection PE (PE2) or the DNI-PW, as both the protection PW (PW2)
and the AC connecting to PE2 are in standby state.
If a failure occurs in AC1, the state of AC2 changes to active
according to the AC redundancy mechanism, while there is no change in
the state of the working and protection PWs. According to the
forwarding state machine in Table 1, PE1 starts to forward traffic
between the working PW and the DNI-PW, and PE2 starts to forward
traffic between AC2 and the DNI-PW. It should be noted that in this
case only AC switchover takes place; in the PSN, traffic is still
forwarded using the working PW.
If a failure in the PSN brings PW1 down, the failure can be detected
by PE1 or PE3 using existing OAM mechanisms. If PE1 detects the
failure of PW1, it MUST inform PE2 of the state of the working PW
using the PW Status TLV in the DHC messages and change the forwarding
status of PW1 to standby. On receipt of the DHC message, PE2 SHOULD
change the forwarding status of PW2 to active. Then, according to
the forwarding state machine in Table 1, PE1 SHOULD set up the
connection between the DNI-PW and AC1, and PE2 SHOULD set up the
connection between PW2 and the DNI-PW. According to the linear
protection mechanism [RFC6378], PE2 also sends an appropriate
protection coordination message [RFC6378] over the protection PW
(PW2) to PE3 for the remote side to switchover from PW1 to PW2. If
PE3 detects the failure of PW1, according to the linear protection
mechanism [RFC6378], it sends a protection coordination message on
the protection PW (PW2) to inform PE2 of the failure on the working
PW. Upon receipt of the message, PE2 SHOULD change the forwarding
status of PW2 to active and set up the connection according to the
forwarding state machine in Table 1. PE2 SHOULD send a DHC message
to PE1 with the S bit set in the Dual-Node Switching TLV to
coordinate the switchover on PE1 and PE2. This is useful for a
unidirectional failure that cannot be detected by PE1.
If a failure brings the working PE (PE1) down, the failure can be
detected by both PE2 and PE3 using existing OAM mechanisms. Both PE2
and PE3 SHOULD change the forwarding status of PW2 to active and send
a protection coordination message [RFC6378] on the protection PW
(PW2) to inform the remote side to switchover. According to the
existing AC redundancy mechanisms, the status of AC1 changes to
Cheng, et al. Standards Track [Page 12]
RFC 8185 Dual-Homing Coordination for MPLS-TP PWs June 2017
standby and the state of AC2 changes to active. According to the
forwarding state machine in Table 1, PE2 starts to forward traffic
between the PW2 and AC2.
5. IANA Considerations
IANA has assigned a new channel type for the "MPLS-TP Dual-Homing
Coordination Message" from the "MPLS Generalized Associated Channel
(G-ACh) Types (including Pseudowire Associated Channel Types)"
subregistry within the "Generic Associated Channel (G-ACh)
Parameters" registry.
Value Description Reference
0x0009 MPLS-TP Dual-Homing Coordination message RFC 8185
IANA has created a new subregistry called "MPLS-TP DHC TLVs" within
the "Generic Associated Channel (G-ACh) Parameters" registry. The
registry has the following fields and initial allocations:
Type Description Length Reference
0x0000 Reserved
0x0001 PW Status 20 Bytes RFC 8185
0x0002 Dual-Node Switching 16 Bytes RFC 8185
The allocation policy for this registry is IETF Review, as specified
in [RFC8126].
6. Security Considerations
MPLS-TP is a subset of MPLS and so builds upon many of the aspects of
the MPLS security model. Please refer to [RFC5920] for generic MPLS
security issues and methods for securing traffic privacy and
integrity.
The DHC message defined in this document contains control
information. If it is injected or modified by an attacker, the dual-
homing PEs might not agree on which PE should be used to deliver the
CE traffic, and this could be used as a denial-of-service attack
against the CE. It is important that the DHC message be used within
a trusted MPLS-TP network domain as described in [RFC6941].
The DHC message is carried in the G-ACh [RFC5586], so it is dependent
on the security of the G-ACh itself. The G-ACh is a generalization
of the Associated Channel defined in [RFC4385]. Thus, this document
relies on the security mechanisms provided for the Associated Channel
as described in those two documents.
Cheng, et al. Standards Track [Page 13]
RFC 8185 Dual-Homing Coordination for MPLS-TP PWs June 2017
As described in the Security Considerations section of [RFC6378], the
G-ACh is essentially connection oriented, so injection or
modification of control messages requires the subversion of a transit
node. Such subversion is generally considered hard in connection-
oriented MPLS networks and impossible to protect against at the
protocol level. Management-level techniques are more appropriate.
The procedures and protocol extensions defined in this document do
not affect the security model of MPLS-TP linear protection as defined
in [RFC6378].
Uniqueness of the identifiers defined in this document is guaranteed
by the assigner (e.g., the operator). Failure by an assigner to use
unique values within the specified scoping for any of the identifiers
defined herein could result in operational problems. Please refer to
[RFC6370] for more details about the uniqueness of the identifiers.
7. References
7.1. Normative References
[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>.
[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>.
[RFC5586] Bocci, M., Ed., Vigoureux, M., Ed., and S. Bryant, Ed.,
"MPLS Generic Associated Channel", RFC 5586,
DOI 10.17487/RFC5586, June 2009,
<http://www.rfc-editor.org/info/rfc5586>.
[RFC6370] Bocci, M., Swallow, G., and E. Gray, "MPLS Transport
Profile (MPLS-TP) Identifiers", RFC 6370,
DOI 10.17487/RFC6370, September 2011,
<http://www.rfc-editor.org/info/rfc6370>.
[RFC6378] Weingarten, Y., Ed., Bryant, S., Osborne, E., Sprecher,
N., and A. Fulignoli, Ed., "MPLS Transport Profile (MPLS-
TP) Linear Protection", RFC 6378, DOI 10.17487/RFC6378,
October 2011, <http://www.rfc-editor.org/info/rfc6378>.
Cheng, et al. Standards Track [Page 14]RFC 8185 Dual-Homing Coordination for MPLS-TP PWs June 2017
[RFC7271] Ryoo, J., Ed., Gray, E., Ed., van Helvoort, H.,
D'Alessandro, A., Cheung, T., and E. Osborne, "MPLS
Transport Profile (MPLS-TP) Linear Protection to Match the
Operational Expectations of Synchronous Digital Hierarchy,
Optical Transport Network, and Ethernet Transport Network
Operators", RFC 7271, DOI 10.17487/RFC7271, June 2014,
<http://www.rfc-editor.org/info/rfc7271>.
[RFC7324] Osborne, E., "Updates to MPLS Transport Profile Linear
Protection", RFC 7324, DOI 10.17487/RFC7324, July 2014,
<http://www.rfc-editor.org/info/rfc7324>.
[RFC8077] Martini, L., Ed. and G. Heron, Ed., "Pseudowire Setup and
Maintenance Using the Label Distribution Protocol (LDP)",
STD 84, RFC 8077, DOI 10.17487/RFC8077, February 2017,
<http://www.rfc-editor.org/info/rfc8077>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <http://www.rfc-editor.org/info/rfc8174>.
7.2. Informative References
[RFC4385] Bryant, S., Swallow, G., Martini, L., and D. McPherson,
"Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for
Use over an MPLS PSN", RFC 4385, DOI 10.17487/RFC4385,
February 2006, <http://www.rfc-editor.org/info/rfc4385>.
[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>.
[RFC6372] Sprecher, N., Ed. and A. Farrel, Ed., "MPLS Transport
Profile (MPLS-TP) Survivability Framework", RFC 6372,
DOI 10.17487/RFC6372, September 2011,
<http://www.rfc-editor.org/info/rfc6372>.
[RFC6718] Muley, P., Aissaoui, M., and M. Bocci, "Pseudowire
Redundancy", RFC 6718, DOI 10.17487/RFC6718, August 2012,
<http://www.rfc-editor.org/info/rfc6718>.
[RFC6870] Muley, P., Ed. and M. Aissaoui, Ed., "Pseudowire
Preferential Forwarding Status Bit", RFC 6870,
DOI 10.17487/RFC6870, February 2013,
<http://www.rfc-editor.org/info/rfc6870>.
Cheng, et al. Standards Track [Page 15]RFC 8185 Dual-Homing Coordination for MPLS-TP PWs June 2017
[RFC6941] Fang, L., Ed., Niven-Jenkins, B., Ed., Mansfield, S., Ed.,
and R. Graveman, Ed., "MPLS Transport Profile (MPLS-TP)
Security Framework", RFC 6941, DOI 10.17487/RFC6941, April
2013, <http://www.rfc-editor.org/info/rfc6941>.
[RFC7771] Malis, A., Ed., Andersson, L., van Helvoort, H., Shin, J.,
Wang, L., and A. D'Alessandro, "Switching Provider Edge
(S-PE) Protection for MPLS and MPLS Transport Profile
(MPLS-TP) Static Multi-Segment Pseudowires", RFC 7771,
DOI 10.17487/RFC7771, January 2016,
<http://www.rfc-editor.org/info/rfc7771>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<http://www.rfc-editor.org/info/rfc8126>.
[RFC8184] Cheng, W., Wang, L., Li, H., Davari, S., and J. Dong,
"Dual-Homing Protection for MPLS and the MPLS Transport
Profile (MPLS-TP) Pseudowires", RFC 8184,
DOI 10.17487/RFC8184, June 2017.
Contributors
The following individuals substantially contributed to the content of
this document:
Kai Liu
Huawei Technologies
Email: alex.liukai@huawei.com
Shahram Davari
Broadcom Corporation
Email: davari@broadcom.com
Cheng, et al. Standards Track [Page 16]RFC 8185 Dual-Homing Coordination for MPLS-TP PWs June 2017
Authors' Addresses
Weiqiang Cheng
China Mobile
No.32 Xuanwumen West Street
Beijing 100053
China
Email: chengweiqiang@chinamobile.com
Lei Wang
China Mobile
No.32 Xuanwumen West Street
Beijing 100053
China
Email: Wangleiyj@chinamobile.com
Han Li
China Mobile
No.32 Xuanwumen West Street
Beijing 100053
China
Email: Lihan@chinamobile.com
Jie Dong
Huawei Technologies
Huawei Campus, No. 156 Beiqing Rd.
Beijing 100095
China
Email: jie.dong@huawei.com
Alessandro D'Alessandro
Telecom Italia
via Reiss Romoli, 274
Torino 10148
Italy
Email: alessandro.dalessandro@telecomitalia.it
Cheng, et al. Standards Track [Page 17]
