Switching Provider Edge (S-PE) Protection for MPLS and MPLS Transport Profile (MPLS-TP) Static Multi-Segment Pseudowires
How static multi-segment pseudowires survive the failure of a switching router on their path.
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, <https://www.rfc-editor.org/info/rfc7771>.
@misc{rfc7771,
series = {Request for Comments},
number = 7771,
howpublished = {RFC 7771},
publisher = {RFC Editor},
doi = {10.17487/RFC7771},
url = {https://www.rfc-editor.org/info/rfc7771},
author = {A. Malis and L. Andersson and H. van Helvoort and J. Shin and L. Wang and A. D'Alessandro},
title = {{Switching Provider Edge (S-PE) Protection for MPLS and MPLS Transport Profile (MPLS-TP) Static Multi-Segment Pseudowires}},
pagetotal = 9,
year = 2016,
month = jan,
} RFC text
Contents
Internet Engineering Task Force (IETF) A. Malis, Ed.
Request for Comments: 7771 L. Andersson
Updates: 6870 Huawei Technologies Co., Ltd.
Category: Standards Track H. van Helvoort
ISSN: 2070-1721 Hai Gaoming BV
J. Shin
SK Telecom
L. Wang
China Mobile
A. D'Alessandro
Telecom Italia
January 2016
Switching Provider Edge (S-PE) Protection for MPLS and MPLS Transport
Profile (MPLS-TP) Static Multi-Segment Pseudowires
Abstract
In MPLS and MPLS Transport Profile (MPLS-TP) environments, statically
provisioned Single-Segment Pseudowires (SS-PWs) are protected against
tunnel failure via MPLS-level and MPLS-TP-level tunnel protection.
With statically provisioned Multi-Segment Pseudowires (MS-PWs), each
segment of the MS-PW is likewise protected from tunnel failures via
MPLS-level and MPLS-TP-level tunnel protection. However, static MS-
PWs are not protected end-to-end against failure of one of the
Switching Provider Edge Routers (S-PEs) along the path of the MS-PW.
This document describes how to achieve this protection via redundant
MS-PWs by updating the existing procedures in RFC 6870. It also
contains an optional approach based on MPLS-TP Linear 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 5741.
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/rfc7771.
Copyright Notice
Copyright (c) 2016 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
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Extension to RFC 6870 to Protect Statically Provisioned
SS-PWs and MS-PWs . . . . . . . . . . . . . . . . . . . . . . 3
3. Operational Considerations . . . . . . . . . . . . . . . . . 5
4. Security Considerations . . . . . . . . . . . . . . . . . . . 5
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
5.1. Normative References . . . . . . . . . . . . . . . . . . 5
5.2. Informative References . . . . . . . . . . . . . . . . . 6
Appendix A. Optional Linear Protection Approach . . . . . . . . 7
A.1. Introduction . . . . . . . . . . . . . . . . . . . . . . 7
A.2. Encapsulation of the PSC Protocol for Pseudowires . . . . 8
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
In MPLS and MPLS Transport Profile (MPLS-TP) Packet Switched Networks
(PSNs), pseudowires (PWs) are transported by MPLS(-TP) Label Switched
Paths (LSPs), also known as tunnels.
As described in RFC 5659 [RFC5659], Multi-Segment Pseudowires (MS-
PWs) consist of Terminating Provider Edge Routers PEs (T-PEs), one or
more Switching Provider Edge Routers (S-PEs), and a sequence of
tunneled PW segments that connects one of the T-PEs with its
"adjacent" S-PE, connects this S-PE with the next S-PE in the
sequence, and so on until the last S-PE is connected by the last PW
segment to the remaining T-PE. In MPLS and MPLS-TP environments,
statically provisioned Single-Segment Pseudowires (SS-PWs) are
protected against tunnel failure via MPLS-level and MPLS-TP-level
tunnel protection. With statically provisioned Multi-Segment
Pseudowires (MS-PWs), each PW segment of the MS-PW is likewise
protected from tunnel failure via MPLS-level and MPLS-TP-level tunnel
protection. However, tunnel protection does not protect static MS-
PWs from failures of S-PEs along the path of the MS-PW.
RFC 6718 [RFC6718] provides a general framework for PW protection,
and RFC 6870 [RFC6870], which is based upon that framework, describes
protection procedures for MS-PWs that are dynamically signaled using
LDP. This document describes how to achieve protection against S-PE
failure in a static MS-PW by extending RFC 6870 to be applicable for
statically provisioned MS-PWs pseudowires (PWs) as well.
This document also contains an OPTIONAL alternative approach based on
MPLS-TP Linear Protection. This approach, described in Appendix A,
MUST be identically provisioned in the PE endpoints for the protected
MS-PW in order to be used. See Appendix A for further details on
this alternative approach.
This document differs from [PW-REDUNDANCY] in that it provides end-
to-end resiliency for static MS-PWs, whereas [PW-REDUNDANCY] provides
resiliency at intermediate S-PEs and resiliency for both dynamically
signaled and static MS-PWs.
PWs based on the Layer 2 Tunneling Protocol Version 3 (L2TPv3) are
outside the scope of this document.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. Extension to RFC 6870 to Protect Statically Provisioned SS-PWs and
MS-PWs
Section 3.2.3 of RFC 6718 and Appendix A.5 of RFC 6870 document how
to use redundant MS-PWs to protect an MS-PW against S-PE failure in
the case of a singly homed Customer Edge (CE), using the following
network model from RFC 6718:
Native |<----------- Pseudowires ----------->| Native
Service | | Service
(AC) | |<-PSN1-->| |<-PSN2-->| | (AC)
| V V V V V V |
| +-----+ +-----+ +-----+ |
+----+ | |T-PE1|=========|S-PE1|=========|T-PE2| | +----+
| |-------|......PW1-Seg1.......|.PW1-Seg2......|-------| |
| CE1| | |=========| |=========| | | CE2|
| | +-----+ +-----+ +-----+ | |
+----+ |.||.| |.||.| +----+
|.||.| +-----+ |.||.|
|.||.|=========| |========== .||.|
|.||...PW2-Seg1......|.PW2-Seg2...||.|
|.| ===========|S-PE2|============ |.|
|.| +-----+ |.|
|.|============+-----+============= .|
|.....PW3-Seg1.| | PW3-Seg2......|
==============|S-PE3|===============
| |
+-----+
Figure 1: Single-Homed CE with Redundant MS-PWs
In this figure, Customer Edge Router 1 (CE1) is connected to T-PE1,
and CE2 is connected to T-PE2 via Attachment Circuits (ACs). There
are three MS-PWs. PW1 is switched at S-PE1, PW2 is switched at
S-PE2, and PW3 is switched at S-PE3. This scenario provides N:1
protection against S-PE failure for the subset of the path of the
emulated service from T-PE1 to T-PE2.
The procedures in RFCs 6718 and 6870 rely on LDP-based PW status
signaling to signal the state of the primary MS-PW that is being
protected, and the precedence in which redundant MS-PW(s) should be
used to protect the primary MS-PW should it fail. These procedures
make use of information carried by the PW Status TLV, which, for
dynamically signaled PWs, is carried by the LDP.
However, statically provisioned PWs (SS-PWs or MS-PWs) do not use the
LDP for PW setup and signaling; rather, they are provisioned by
network management systems or other means at each T-PE and S-PE along
their paths. They also do not use the LDP for status signaling.
Rather, they use procedures defined in RFC 6478 [RFC6478] for status
signaling via the PW Operations, Administration, and Maintenance
(OAM) message using the PW Associated Channel Header (ACH). The PW
Status TLV carried via this status signaling is itself identical to
the PW Status TLV carried via LDP-based status signaling, including
the identical PW Status Codes.
Sections 6 and 7 of RFC 6870 describe the management of a primary PW
and its secondary PW(s) to provide resiliency to the failure of the
primary PW. They use status codes transmitted between endpoint T-PEs
using the PW Status TLV transmitted by LDP. For this management to
apply to statically provisioned PWs, the PW status signaling defined
in RFC 6478 MUST be used for the primary and secondary PWs. In that
case, the endpoint T-PEs can then use the PW status signaling
provided by RFC 6478 in place of LDP-based status signaling, so that
the status-signaling-based procedures in RFC 6870 operate identically
to when used with LDP-based status signaling. Note that the optional
S-PE Bypass Mode defined in Section 5.5 of RFC 6478 cannot be used,
as it requires LDP signaling.
3. Operational Considerations
Because LDP is not used between the T-PEs for statically provisioned
MS-PWs, the negotiation procedures described in RFC 6870 cannot be
used. Thus, operational care must be taken so that the endpoint
T-PEs are identically provisioned regarding the use of this document,
specifically whether or not MS-PW redundancy is being used, and for
each protected MS-PW, the identity of the primary MS-PW and the
precedence of the secondary MS-PWs.
4. Security Considerations
The security considerations defined for RFC 6478 apply to this
document as well. As the security considerations in RFCs 6718 and
6870 are related to their use of LDP, they are not required for this
document.
If the alternative approach in Appendix A is used, then the security
considerations defined for RFCs 6378, 7271, and 7324 also apply.
5. References
5.1. 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>.
[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>.
[RFC6478] Martini, L., Swallow, G., Heron, G., and M. Bocci,
"Pseudowire Status for Static Pseudowires", RFC 6478,
DOI 10.17487/RFC6478, May 2012,
<http://www.rfc-editor.org/info/rfc6478>.
[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>.
[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>.
5.2. Informative References
[PW-REDUNDANCY]
Dong, J. and H. Wang, "Pseudowire Redundancy on S-PE",
Work in Progress, draft-ietf-pals-redundancy-spe-02,
August 2015.
[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>.
[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>.
Appendix A. Optional Linear Protection Approach
A.1. Introduction
In "MPLS Transport Profile (MPLS-TP) Linear Protection" [RFC6378], as
well as in the later updates of that RFC "MPLS Transport Profile
(MPLS-TP) Linear Protection to Match the Operational Expectations of
Synchronous Digital Hierarchy, Optical Transport Network, and
Ethernet Transport Network Operators" [RFC7271] and "Updates to MPLS
Transport Profile Linear Protection" [RFC7324], the Protection State
Coordination (PSC) protocol was defined for MPLS LSPs only.
This appendix extends these RFCs to be applicable for PWs (SS-PW and
MS-PW) as well. This is useful especially in the case of end-to-end
static provisioned MS-PWs running over MPLS-TP where tunnel
protection alone cannot be relied upon for end-to-end protection of
PWs against S-PE failure. It also enables a uniform operational
approach for protection at LSP and PW layers and an easier management
integration for networks that already implement the approach in RFCs
6378, 7271, and 7324.
The protection architectures are those defined in [RFC6378]. For the
purposes of this appendix, we define the protection domain of a
point-to-point PW as consisting of two terminating PEs (T-PEs) and
the transport paths that connect them (see Figure 2).
+-----+ //=======================\\ +-----+
|T-PE1|// Working Path \\|T-PE2|
| /| |\ |
| ?< | | >? |
| \| |/ |
| |\\ Protection Path //| |
+-----+ \\=======================// +-----+
|<-------Protection Domain------->|
Figure 2: Protection Domain
This Appendix is an OPTIONAL alternative approach to the one in
Section 2. For interoperability, all implementations MUST include
the approach in Section 2, even if this alternative approach is used.
The operational considerations in Section 3 continue to apply when
this approach is used, and operational care must be taken so that the
endpoint T-PEs are identically provisioned regarding the use of this
document.
A.2. Encapsulation of the PSC Protocol for Pseudowires
The PSC protocol can be used to protect against defects on any LSP
(segment, link, or path). In the case of MS-PW, the PSC protocol can
also protect failed intermediate nodes (S-PE). Linear protection
protects an LSP or PW end-to-end and if a failure is detected,
switches traffic over to another (redundant) set of resources.
Obviously, the protected entity does not need to be of the same type
as the protecting entity. For example, it is possible to protect a
link by a path. Likewise, it is possible to protect an SS-PW with an
MS-PW, and vice versa.
From a PSC protocol point of view, it is possible to view an SS-PW as
a single-hop LSP and an MS-PW as a multiple-hop LSP. Thus, this
provides end-to-end protection for the SS-PW or MS-PW. The Generic
Associated Channel (G-Ach) carrying the PSC protocol information is
placed in the label stack directly beneath the PW identifier. The
PSC protocol will then work as specified in RFCs 6378, 7271, and
7324.
Acknowledgements
The authors would like to thank Matthew Bocci, Yaakov Stein, David
Sinicrope, Sasha Vainshtein, and Italo Busi for their comments on
this document.
Figure 1 and the explanatory paragraph following the figure were
taken from RFC 6718. Figure 2 was adapted from RFC 6378.
Authors' Addresses
Andrew G. Malis (editor)
Huawei Technologies Co., Ltd.
Email: agmalis@gmail.com
Loa Andersson
Huawei Technologies Co., Ltd.
Email: loa@mail01.huawei.com
Huub van Helvoort
Hai Gaoming BV
Email: huubatwork@gmail.com
Jongyoon Shin
SK Telecom
Email: jongyoon.shin@sk.com
Lei Wang
China Mobile
Email: wangleiyj@chinamobile.com
Alessandro D'Alessandro
Telecom Italia
Email: alessandro.dalessandro@telecomitalia.it
AI ELI5
On this page
§1The Problem: S-PE Failure
In MPLS and MPLS Transport Profile (MPLS-TP) networks, pseudowires (PWs) are carried by Label Switched Paths (LSPs), also called tunnels. A Multi-Segment Pseudowire (MS-PW) is a chain: a Terminating Provider Edge router (T-PE) at each end, one or more Switching Provider Edge routers (S-PEs) in between, and a tunneled PW segment joining each neighbouring pair.
Statically provisioned single-segment PWs and each segment of a static MS-PW are already protected against tunnel failure by MPLS-level and MPLS-TP-level tunnel protection. Tunnel protection does not help when an S-PE along the path fails. That gap is what this RFC closes.
The RFC is based on the general PW protection framework in RFC 6718 and on RFC 6870, which describes protection for MS-PWs that are signaled dynamically with LDP. L2TPv3-based PWs are out of scope.
A road already has a spare lane if one bridge breaks. But nobody had planned for what happens if a whole stop in the middle of the trip closes. This document plans for that.
§2Redundant MS-PWs in the Network Model
The RFC reuses the network model from RFC 6718, where a singly homed Customer Edge router (CE) is connected to a T-PE through an Attachment Circuit. In the figure there are three MS-PWs between T-PE1 and T-PE2: PW1 is switched at S-PE1, PW2 at S-PE2 and PW3 at S-PE3.
Having several MS-PWs through different S-PEs gives N:1 protection against S-PE failure for the part of the path from T-PE1 to T-PE2. If the S-PE on the PW in use fails, one of the others can take over.
There are three different routes from one end to the other, each going through a different middle stop. If one stop closes, you take another route.
§3Extending RFC 6870 to Static Pseudowires
The procedures in RFC 6718 and RFC 6870 rely on LDP-based PW status signaling. It carries the state of the primary MS-PW and the order in which redundant MS-PWs should be used, inside the PW Status TLV.
Static PWs do not use LDP for setup or status. They are provisioned at each T-PE and S-PE by network management systems or other means, and they signal status with the PW OAM message over the PW Associated Channel Header, as defined in RFC 6478. The PW Status TLV carried this way is identical to the LDP one, including the PW Status Codes.
So, for the management of a primary PW and its secondary PWs in Sections 6 and 7 of RFC 6870 to work on static PWs, the status signaling of RFC 6478 MUST be used for the primary and secondary PWs. The endpoint T-PEs then use it in place of LDP, and the RFC 6870 procedures operate identically. The optional S-PE Bypass Mode of RFC 6478 cannot be used, because it requires LDP signaling.
The two end kids used to talk over one kind of phone line. Static pipes have no such line, so they use a different one that carries exactly the same words.
| Aspect | Dynamic MS-PW (RFC 6870) | Static MS-PW (this RFC) |
|---|---|---|
| Setup | Signaled with LDP | Provisioned at each T-PE and S-PE |
| Status signaling | PW Status TLV over LDP | PW Status TLV via PW OAM message (RFC 6478) |
| Negotiation between T-PEs THIS RFC | Possible | Not possible; provision identically |
§4Provisioning Both Ends Identically
Because LDP is not used between the T-PEs of a static MS-PW, the negotiation procedures of RFC 6870 cannot be used. Operators therefore have to take care that the two endpoint T-PEs are identically provisioned.
Specifically, both ends must agree on whether MS-PW redundancy is in use at all. For each protected MS-PW they must also agree on which MS-PW is the primary and on the precedence of the secondary MS-PWs.
Since the two ends cannot chat to agree on the plan, someone has to give both of them exactly the same written plan beforehand.
§5Security Considerations
The security considerations of RFC 6478 apply to this document. The security considerations of RFC 6718 and RFC 6870 relate to their use of LDP, so they are not required here.
If the alternative approach in the appendix is used, the security considerations of RFC 6378, RFC 7271 and RFC 7324 also apply.
The safety rules of the phone line the ends use also apply here. If you pick the optional method, you follow its extra safety rules too.
§6Optional Linear Protection Approach
Appendix A offers an OPTIONAL alternative based on MPLS-TP Linear Protection. The Protection State Coordination (PSC) protocol of RFC 6378, RFC 7271 and RFC 7324 was defined for MPLS LSPs only, and the appendix extends it to single-segment and multi-segment PWs. It is useful for end-to-end static MS-PWs over MPLS-TP, where tunnel protection alone cannot protect against S-PE failure.
The protection domain of a point-to-point PW is the two T-PEs and the transport paths between them, a working path and a protection path. PSC treats an SS-PW as a single-hop LSP and an MS-PW as a multiple-hop LSP, so it can also protect against a failed intermediate S-PE. The protected entity does not need to be of the same type as the protecting one, so an SS-PW can be protected by an MS-PW and vice versa.
The Generic Associated Channel (G-ACh) that carries the PSC information is placed in the label stack directly beneath the PW identifier. This approach MUST be identically provisioned at the PE endpoints. For interoperability, all implementations MUST include the approach of the main text even when this alternative is used.
There is a second, optional way: the two ends keep a main path and a spare path and keep telling each other which one to use. Both ends must be set up the same way.