Signal Degrade Indication in
Segment Routing over MPLS NetworkChina MobileBeijingChinahanliuyan@chinamobile.comHuawei TechnologiesBeijingChinashirley.yangfan@huawei.comCAICTBeijingChinazhaojunfeng@caict.ac.cnMPLS Working GroupSignal Degrade Indication, Segment Routing, SRThis 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.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.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.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.MPLS: MultiProtocol Label SwitchingPWE3: Pseudo Wire Emulation Edge to EdgeMPLS-TP: MultiProtocol Label Switching - Transport ProfileSR: Segment RoutingSDN: Software Defined NetworkOAM: Operation, Administration and MaintenanceSD: Signal DegradeBER: Bit Error RateWDM: Wavelength Division MultiplexingNMS: Network Management SystemG-ACh: Generic Associated ChannelPDU: Protocol Data UnitCCM: Continuity Check MessageMEP: Maintenance Entity Group End PointMIP: Maintenance Entity Group Intermediate PointAIS: Alarm Indication SignalSignal 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.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 , 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 .Generic Associated Channel (G-ACh) 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.In MPLS-TP, 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.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.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.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.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.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.RDI: Remote Defect Indication, set to 1 to indicate RDI,
otherwise it is set to 0.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.This document requests IANA to assign one bit from Flags of MPLS-TP
OAM PDU format to indicate "Signal Degrade".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.The authors want to thank Yuanlong Jiang, Mach Chen, Yongjian Hu for
their valuable suggestions during the construction of draft.G.8113.1: Operations, administration and maintenance
mechanisms for MPLS-TP in packet transport networksITU-T