Multi-Layer OAM for Service Function Chains in Networks ZTE Corp.gregimirsky@gmail.comZTE CorporationNo.50 Software Avenue, Yuhuatai DistrictNanjingChinameng.wei2@zte.com.cn,vally.meng@gmail.comZTE TX, Inc.55 Madison Avenue, Suite 160Morristown, New Jersey 07960USAbhumip.khasnabish@ztetx.comlindawangjoy@gmail.com
Routing
SFC WGRequest for CommentsRFCInternet DraftI-D
A multi-layer approach to the task of Operation, Administration
and Maintenance (OAM) of Service Function Chains (SFCs) in networks is presented.
Based on the SFC OAM requirements, a multi-layer model is introduced.
A mechanism to detect and localize defects using the multi-layer model
is also described.
defines components necessary to implement
Service Function Chain (SFC). These include a classifier
which performs classification of incoming packets. A Service Function Forwarder (SFF)
is responsible for forwarding traffic to one or more connected Service Functions (SFs) according to
the information carried in the SFC encapsulation. SFF also handles traffic coming back from
the SF and transports the data packets to the next SFF. And the SFF serves as termination
element of the Service Function Path (SFP). SF is responsible for specific treatment
of received packets.
Resulting from that SFC is constructed by a number of these components,
there are different views from different levels of the SFC.
One is the SFC, fully abstract entity, that defines an ordered set of SFs that must
be applied to packets selected as a result of classification.
But SFC doesn't define exact mapping between SFFs and SFs. Thus there exists another
semi-abstract entity referred as SFP. SFP is
the instantiation of the SFC in the network and provides a level of indirection
between the fully abstract SFC and a fully specified ordered
list of SFFs and SFs identities that the packet will visit when
it traverses the SFC. The latter entity is being referred as Rendered Service Path (RSP).
The main difference between SFP and RSP is that in the former the authority to
select the SFF/SF has been delegated to the network.
This document proposes the multi-layer model of SFC Operation, Administration
and Maintenance (OAM) and requirements to improve the troubleshooting efficiency.
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
when, and only when, they appear in all capitals, as shown here.
Unless explicitly specified in this document, active OAM in SFC and SFC OAM
are being used interchangeably.
e2e: End-to-EndFM: Fault ManagementOAM: Operations, Administration, and MaintenanceRDI: Remote Defect IndicationRSP: Rendered Service PathSF: Service FunctionSFC: Service Function ChainSFF: Service Function ForwarderSFP: Service Function Path
As described in , multiple
layers come into play to realize the SFC,
including the Service layer, the underlying Network layer, as well as the Link layer,
which are depicted in :
The Service layer consists of classifiers and/or service functions/SFs. Network and Transport layers leverage various overlay network technologies
interconnecting SFs to establish SFP.The Link layer is technology specific and reflects the technology used in
the underlay network.
To perfrom the OAM task of fault management (FM) in an SFC, that inculdes failure detection, defect
characterzation and localization, this document defines the
multi-layer model of OAM, presented in , and set of requirements
towards active OAM mechanisms to be used on an SFC.
In example presented in the service SFP1 may be realized
through two RSPs, RSP1(SF1--SF3--SF5) and RSP2(SF2--SF4--SF6). To perform
end-to-end (e2e) FM SFC OAM:
REQ#1: Packets of active OAM in SFC SHOULD be fate sharing with data traffic,
i.e. in-band with the monitored traffic, i.e. follow exactly
the same RSP, in forward direction, i.e. from ingress toward egress end
point(s) of the OAM test.
REQ#2: SFC OAM MUST support pro-active monitoring of any element in the SFC
availability.
The egress, SFF3 in example in ,
is the entity that detects the failure of the SFC. It must
be able to signal the new defect state to the ingress, i.e. SFF1.
Hence the following requirement:
REQ#3: SFC OAM MUST support Remote Defect Indication (RDI)
notification by egress to the ingress, i.e. source of
continuity checking.
REQ#4: SFC OAM MUST support connectivity verification.
Definition of mis-connectivity defect entry and exit
criteria are outside the scope of this document.
Once the SFF1 detects the defect objective of OAM switches from failure detection
to defect characterization and localization.
REQ#5: SFC OAM MUST support fault localization of Loss of
Continuity check in the SFC.
REQ#6: SFC OAM MUST support tracing an SFP in order to realize the RSP.
It is practical, as presented in , that several SFs share the same SFF.
In such case SFP1 may be realized over two RSPs, RSP1(SF1--SF3--SF5) and RSP2(SF2--SF4--SF6).
REQ#7: SFC OAM MUST have the ability to discover and exercise
all available RSPs in the transport network.
In process of localizing the SFC failure separating SFC OAM layers is very attractive and efficient approach.
To achieve that continuity among SFFs that are part of the same SFP should be verified. Once SFFs reacheability
along the particular SFP has been confirmed task of defect localization may focus on SF reacheability verification.
Because reacheability of SFFs has already been verified, SFF local to the SF may be used as source.
REQ#8: SFC OAM MUST be able to trigger on-demand FM with
responses being directed towards initiator of such proxy
request.
By using the multi-layer model OAM that confirms to the above listed requirements is capable to
perform efficient defect localization on an SFC.
presents a use case of applying the proposed SFC OAM multi-layer model.
In this scenario operator needs to discover SFFs and SFs of the same SFC.
The Layer 1 includes the SFFs that are part of the SFP. The Layer 2 - the SFs along the RSP.
When trying to do SFC OAM, classifier or service nodes select and
confirm which SFC OAM layering they plan to do,
then encapsulate the layering information in the SFC OAM packets, and
send the SFC OAM packets along the service function paths
to the destination. When receiving the SFC OAM packets, service nodes
analyze the layering information and then decide
whether sending these packets to next SFFs directly without being processed
by SFs for Layer 1 process or sending to SFs for Layer 2 process.
Echo Request/Reply is well-known OAM mechanism that is extecively used to detect inconsitencies between states in control plane
and data plane, localize defects in the data plane. In SFC OAM Echo Request/Reply is built as extension of Overlay
Echo Request/Reply functions .
Responder to the SFC Echo Request sends the Echo Reply over IP network if the reply mode is
Reply via an IPv4/IPv6 UDP Packet .
Because SFC NSH does not identify the ingress of the SFP
the Echo Request MUST include this information that to be used as IP destination address for IP/UDP encapsulation of the SFC Echo Reply.
Sender of the SFC Echo Request MUST include SFC Source TLV .
where
SFC OAM Source Id Type is two octets in length and has the value of TBD1 .
Length is two octets long field and the valuse is equal to the length of the Value field.
Value field contains IP address of the sender of the SFC OAM control message, IPv4 or IPv6.
The UDP destination port for SFC Echo Reply
TBD2 will be allocated by IANA .
TBD
IANA is requested to create SFC OAM TLV Type registry.
All code points in the range 1 through 32759 in this registry shall be allocated
according to the "IETF Review" procedure as specified in .
Code points in the range
32760 through 65279 in this registry shall be allocated according to the "First Come First Served" procedure as
specified in .
Remaining code points are allocated according to the :
ValueDescriptionReference0ReservedThis document1- 32759UnassignedIETF Review32760 - 65279UnassignedFirst Come First Served65280 - 65519ExperimentalThis document65520 - 65534Private UseThis document65535ReservedThis document This document defines the following new value in SFC OAM TLV Type registry:ValueDescriptionReferenceTBD1Source IP AddressThis document
IANA is requested to allocate UDP port number according to
Service Name Port Number Transport Protocol Description Semantics Definition Reference SFC OAMTBD2UDPSFC OAMThis document