Internet DRAFT - draft-han-mpls-sdi-sr
draft-han-mpls-sdi-sr
MPLS Working Group L. Han
Internet-Draft China Mobile
Intended status: Standards Track F. Yang
Expires: 29 June 2024 R. Tan
Huawei Technologies
J. Zhao
CAICT
27 December 2023
Signal Degrade Indication in Segment Routing over MPLS Network
draft-han-mpls-sdi-sr-05
Abstract
This document describes a typical use case of MPLS-TP, where signal
degrade defect needs to be correctly detected and transmitted via OAM
messages within network. When MPLS-TP evolves to Segment Routing
MPLS, transit node has no knowledge of labels to be encapsulated in
MPLS label stack. Transit node cannot spread OAM messages with
signal degrade defect indication. Thus, a solution is proposed in
this draft.
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].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on 29 June 2024.
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Copyright Notice
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Table of Contents
1. Background . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Defect Triggered Procedure . . . . . . . . . . . . . . . 4
3.2. MPLS-TP Solution . . . . . . . . . . . . . . . . . . . . 4
3.3. Problem in SR-MPLS . . . . . . . . . . . . . . . . . . . 6
4. Solution in SR-MPLS . . . . . . . . . . . . . . . . . . . . . 6
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1. Normative References . . . . . . . . . . . . . . . . . . 8
8.2. Informative References . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Background
In early era of telecommunication, transport network is set up to
provide voice service. The connection in network is always
connection-oriented and circuit switching. With the rapid increasing
bandwidth brought by Ethernet, transport network transforms into the
packet-switched transport network. Technologies like MPLS/PWE3
perfectly meet the requirements of supporting both packet-transport
and circuit-transport. It led to the work of MPLS Transport Profile
(MPLS-TP), collaborated between ITU-T and IETF at the first decade of
the 21st century.
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MPLS-TP is a subset of MPLS. Features that are not applicable to
transport network are excluded, and features to meet the requirements
of transport network, e.g., bidirectional path, deterministic control
and management, etc., are strictly required. According to the Joint
Working Team consensus, any extension of MPLS-TP would be included in
MPLS field.
With the emerge of Segment Routing (SR) and Software Defined Network
(SDN), MPLS-TP network technologies are adapted as well. In this
draft, we recognize one use case where the signal degrade defect can
be correctly detected and transmitted via MPLS-TP OAM in MPLS-TP, but
not fulfilled in SR-MPLS. To fix this problem is the motivation of
this draft.
Editor's note: This section gives a historical introduction of MPLS-
TP, since it has been extensively deployed in packet switched
transport networks for years. The intention of this section is to
help readers understand the unique of requirements from packet
transport network. Once the draft becomes RFC, part of this section
can be moved to Appendix.
2. Terminology
MPLS: MultiProtocol Label Switching
PWE3: Pseudo Wire Emulation Edge to Edge
MPLS-TP: MultiProtocol Label Switching - Transport Profile
SR: Segment Routing
SDN: Software Defined Network
OAM: Operation, Administration and Maintenance
SD: Signal Degrade
BER: Bit Error Rate
WDM: Wavelength Division Multiplexing
NMS: Network Management System
G-ACh: Generic Associated Channel
PDU: Protocol Data Unit
CCM: Continuity Check Message
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MEP: Maintenance Entity Group End Point
MIP: Maintenance Entity Group Intermediate Point
AIS: Alarm Indication Signal
3. Problem Statement
3.1. Defect Triggered Procedure
Signal Degrade (SD) describes a status of signal associated data has
degraded and a degraded defect is active. Signal degrade of a
physical link is usually measured and represented by Bit Error Rate
(BER) value. Fiber aging, impairment and pollution, optical module
mismatch or WDM transmission error are the reasons to lead to signal
degrade. More information about signal degrade can be found
in[I-D.yang-mpls-ps-sdi-sr].
In practice, when physical link degrades in network, signal degrade
defect is firstly detected and reported by the node. A specific type
of alarm is generated and sent to Network Management System (NMS) or
a SDN controller. It is a report to management plane and strongly
required from perspective of network management. However, the
problem is the notification to management plane is usually not fast
enough to assist the network recovery. It may result in hour or even
day level of service interruption time.
As mentioned in [RFC6372], defect may trigger system to perform a
survivability action, when notification of an issue is reported from
equipment in a lower layer, system fails to receive an OAM continuity
check message, or receives of an OAM message reporting a failure
condition. Similarly, when signal degrade defect is reported from
the lower layer, e.g. physical layer, local protection mechanism can
be triggered within the internal system of nodez. In case of
protection switchover selector is at the source or destination node,
while the signal degrade is happened at intermediate node, an OAM
message should be transmitted to notify the degrade condition to the
nodes actually perform the protection switchover. This action is
preferred to be triggered by events in the data plane [RFC6372].
3.2. MPLS-TP Solution
Generic Associated Channel (G-ACh) [RFC5586] is defined to carry OAM
messages for MPLS pseudowires, LSPs and sections. The Generic
Associated Channel format used in MPLS is shown in Figure 1. By
using the generic associated channel and indication of channel type,
different OAM mechanisms with different formats can be encapsulated
uniformly as well as independently.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Label | EXP |S| TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| GAL Label (13) | TC |S| TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 1|Version| Reserved | Channel Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
In MPLS-TP, ITU-T G.8113.1
Figure 1 G-ACh Format in MPLS
[ITU-T_G.8113.1] specifies a large set of OAM mechanisms and has been
widely deployed in packet transport networks. Figure 2 shows the
common OAM PDU format of different OAM mechanisms.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MEL | Version | OpCode | Flags | TLV Offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OAM PDU |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|End TLV|
+-+-+-+-+
MEG Level: MEG Level is a 3-bit field. It contains an integer value
that identifies the MEG level of OAM PDU. Value ranges from 0 to 7.
Figure 2 ITU-T G.8113.1 Common OAM PDU Format
Version: Version is a 5-bit field. It contains an integer value that
identifies the OAM protocol version. Value is 0 in the current
version.
OpCode: OpCode is a 1-octet field. It contains an OpCode that
identifies an OAM PDU type. OpCode is used to identify the remaining
content of an OAM PDU. Value for the CCM PDU type is 1.
Flags: Flags is an 8-bit field. Use of the bits in this field is
dependent on the OAM PDU type.
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TLV Offset: TLV Offset is a 1-octet field. It contains the offset to
the first TLV in an OAM PDU relative to the TLV Offset field. The
value of this field is associated with an OAM PDU type. When the TLV
Offset is 0, it points to the first octet following the TLV Offset
field.
End TLV: an all-ZEROes octet value.
When signal degrade happens in MPLS-TP, an MPLS-TP Alarm Indication
Signal (AIS) OAM message with active AIS indication is generated and
transmitted within the OAM maintenance domain. Maintenance Entity
Group End Point (MEP), usually also acting as protection switchover
selector, performs the protection switchover once it receives the AIS
indication in MPLS-TP OAM message.
3.3. Problem in SR-MPLS
When Segment Routing is introduced to MPLS, the nodes except the
headend have no information of the forwarding path. If the signal
degrade is happened on the transit nodes, MPLS-TP AIS OAM message
cannot be generated because this node has no knowledge of labels
ought to be encapsulated in MPLS label stack. Either the label
information of forwarding path can be obtained on transit node, or
the defect can be indicated in different messages could help the
defect spread in network. It is valuable to keep transit node with
the capability of reporting defects in SR-MPLS.
4. Solution in SR-MPLS
Segment routing is designed to reduce the states in transit nodes,
any defects like SD defect cannot be indicated in a newly generated
OAM message on transit node. Alternative way is to indicate the
defect in other OAM messages. Continuity Check Message (CCM) is
proposed to indicate the signal degrade defect for two reasons.
Firstly, CCM is designed to be applicable for fault management,
performance monitoring, or protection switching applications.
Secondly, consider the merit of CCM's various transmission period,
the defect indication can be flexibly transmitted according to
operator's needs.
One reservation bits in Flag section in CCM OAM PDU message can be
used as Error Indication (EI) to indicate signal degrade. Flag
format with EI extension is shown in Figure 3.
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MSB LSB
0
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
|RDI| EI| Reserved | Period |
+---+---+---+---+---+---+---+---+
RDI: Remote Defect Indication, set to 1 to indicate RDI, otherwise it
is set to 0.
Figure 3 CCM OAM PDU Flags Format with EI Extension
Period: Indicate the transmission period.
EI: Error indication, 0 indicates no error, 1 indicates error.
Reserved: Reserved fields are set to all ZEROes.
If the node detects the signal degrade defect, EI field is set in CCM
OAM message and transmitted to other nodes. Note that, Maintenance
Entity Group Intermediate Point (MIP) is required to be transparent
to CCM message in MPLS-TP. In order to support BER indication on
each node along the forwarding path, extra configuration and
intervening implementation to process CCM message would be required
on MIP.
Editor's Note: When other OAM mechanisms used in generic associated
channel (G-ACh), there might be various solutions to transmit signal
degrade defect, or any other defects detected by transit nodes. This
draft introduces a very light-weight solution, which has already been
implemented and deployed in networks.
5. IANA Considerations
This document requests IANA to assign one bit from Flags of MPLS-TP
OAM PDU format to indicate "Signal Degrade".
6. Security Considerations
There are MEP and MIP node defined in OAM mechanisms. Some types of
OAM message are defined to be transparent to MIP node, and requires
no extra configuration or message processing on MIP nodes. If the
transit node of SR-MPLS acts as MIP in OAM maintenance domain, this
MIP node needs to process the OAM messages to indicate the defects.
At the moment, explicit configuration is required on MIP to have the
authority to process OAM messages.
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7. Acknowledgements
The authors want to thank Yuanlong Jiang, Mach Chen, Yongjian Hu for
their valuable suggestions during the construction of draft.
8. References
8.1. 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,
<https://www.rfc-editor.org/info/rfc2119>.
8.2. Informative References
[I-D.yang-mpls-ps-sdi-sr]
Yang, F., Han, L., and J. Zhao, "Problem Statement of
Signal Degrade Indication for SR over MPLS", Work in
Progress, Internet-Draft, draft-yang-mpls-ps-sdi-sr-01, 2
November 2020, <https://datatracker.ietf.org/doc/html/
draft-yang-mpls-ps-sdi-sr-01>.
[ITU-T_G.8113.1]
ITU-T, "G.8113.1: Operations, administration and
maintenance mechanisms for MPLS-TP in packet transport
networks", , April 2016.
[RFC5586] Bocci, M., Ed., Vigoureux, M., Ed., and S. Bryant, Ed.,
"MPLS Generic Associated Channel", RFC 5586,
DOI 10.17487/RFC5586, June 2009,
<https://www.rfc-editor.org/info/rfc5586>.
[RFC6372] Sprecher, N., Ed. and A. Farrel, Ed., "MPLS Transport
Profile (MPLS-TP) Survivability Framework", RFC 6372,
DOI 10.17487/RFC6372, September 2011,
<https://www.rfc-editor.org/info/rfc6372>.
Authors' Addresses
Liuyan Han
China Mobile
Beijing,
China
Email: hanliuyan@chinamobile.com
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Fan Yang
Huawei Technologies
Beijing
China
Email: shirley.yangfan@huawei.com
Ren Tan
Huawei Technologies
Beijing
China
Email: tanren@huawei.com
Junfeng Zhao
CAICT
Beijing
China
Email: zhaojunfeng@caict.ac.cn
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