Internet Working Group Y. Jiang
W. Xu
Internet Draft Huawei
Z. Cao
Intended status: Standards Track
Expires: January 2016 July 6, 2015
Fault Management in Service Function Chaining
draft-jxc-sfc-fm-02.txt
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Abstract
This document specifies some Fault Management tools for SFC. It also
describes the procedures of using these tools to perform connectivity
verification and SFP tracing functions upon SFC services.
Table of Contents
1. Introduction .............................................. 2
1.1. Conventions used in this document ...................... 3
1.2. Terminology ............................................ 4
1.3. SFC OAM Requirements ................................... 4
2. Packet Format ............................................. 5
3. Theory of Operation ....................................... 8
3.1. Continuity Check and Connectivity Verification of SFC .. 8
3.1.1. MEP sending an SFC CC-CV packet ..................... 8
3.1.2. MEP terminating an SFC CC-CV packet ................. 9
3.2. SFC Route Tracing ...................................... 9
3.2.1. MEP sending an SFC Trace Request ................... 11
3.2.2. SFF processing an SFC Trace Route Request .......... 11
3.2.3. Service Function treating an SFC Trace Request ..... 11
3.2.4. MEP receiving an SFC Trace Reply ................... 11
4. Security Considerations .................................. 12
5. IANA Considerations ...................................... 12
6. References ............................................... 12
6.1. Normative References .................................. 12
6.2. Informative References ................................ 12
7. Acknowledgments .......................................... 13
1. Introduction
This document discusses Operations, Administration and Maintenance
(OAM), specifically, fault management of Service Function Chaining
(SFC), and further provides a solution that can be used in SFC OAM.
An SFC layer OAM framework was described in [I-D.aldrin-sfc-oam-
framework], which introduced multiple layers of OAM model including
the link layer, the network layer, the service layer. The link layer
and the network layer can leverage existing OAM tool sets available
to perform OAM function, and thus out of scope of this document.
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This document is focused on the SFC OAM mechanism in the service
layer. Currently the service layer does not have OAM tools yet, and
new OAM tools should be developed according to the characteristic of
service layer. In the design of SFC OAM, one important principle is
to reuse existing OAM tools as much as possible. As a result, methods
in this document follow some existing OAM mechanisms, e.g. Ping, BFD,
etc.
A requisite of SFC OAM is that SFC OAM messages must follow the same
data path as normal SFC packets would traverse. This is usually
called "In-band" signaling. SFC OAM tools are used primarily to
validate the SFC data plane, and may further be used to verify the
SFC data plane against the SFC control plane.
SFC OAM tools discussed in this document include Connectivity
Function, Continuity Function, and Trace Function.
SFC Connectivity Function is used to verify the connectivity existing
between classifier or service function and another service function.
Connectivity verification packets will travel along the tested
service chain path to an anticipant endpoint.
SFC Continuity Function is used to validate or verify the constant
reachability for a specific SFP. It can detect failure between a
source device and destination device in a periodic of time, which
usually be used in fast fault detection.
SFC Trace Function is used to locate the fault position. The trace
request packets will travel along the SFP, it will trigger every
transit device to generate response. The trace reply message will
carry information for fault location.
Details of these SFC functions are described in section 3.
1.1. Conventions used in this document
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 [RFC2119].
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1.2. Terminology
Maintenance Entity Group (MEG): The set of one or more maintenance
entities that maintain and monitor a section or a transport path in
an OAM domain.
MEP: MEG End Point, an OAM end point capable of initiating (source
MEP) and terminating (sink MEP) OAM packets for fault management and
performance monitoring.
MIP: MEG Intermediate Point, an OAM intermediate point terminates and
processes OAM packets that are sent to this particular MIP and may
generate OAM packets in reaction to received OAM packets.
Service Chaining Header: a header in front of packet, added by an SFF.
SFF uses service chaining header information to forward service
chaining packet.
Service Chaining Packet: an original packet added with a service
chaining header.
Terminologies introduced in [I-D.ietf-sfc-architecture] will not be
repeated here.
1.3. SFC OAM Requirements
The following SFC OAM requirements MUST be supported:
(R1) SFC OAM MUST allow for continuity check between SFFs.
(R2) SFC OAM MUST allow for connectivity verification between SFFs.
(R3) SFC OAM MUST support trace routing in a service function path.
(R4) SFC OAM MUST support connectivity verification between SFs in an
SFC chain.
(R5) SFC OAM MUST support performance measurements in SFs and SFFs.
(R6) SFC OAM MUST support monitoring of unidirectional and bi-
directional SFC path.
(R7) SFC OAM MUST support fate sharing of SFC OAM packets and SFC
service packets on the same SFC path (congruent path).
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Since control plane is not a prerequisite for SFC, we cannot resort
to control plane hello session. Furthermore, OAM packets need to be
transported on the same data path as the SFC packets, so that any
data plane failure can be identified.
Therefore, there is a need to provide an OAM tool that would enable
users to detect failures in the SFC data plane, and a mechanism to
isolate and identify faults.
This document discusses the fault management problem in SFC. The
basic idea is to verify that packets in a particular Service Function
Chain actually passing through the SFFs and SFs along the respective
SFC path.
It is proposed that this test be carried out by sending an OAM
message (called an "SFC trace request message") across an SFC path.
The SFC trace request message carries the SFC identifier whose SFC
path is being verified. This SFC OAM request message is forwarded on
the SFC path just like any SFC data packet belonging to that Service
Function Chain.
The OAM message is processed by each SFF along the SFC, and the SFF
will respond with an SFC trace Reply message, carrying information
such as the previous SF identifier and its position in the SFC.
2. Packet Format
An SFC OAM payload is depicted in Figure 1.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Version|R|R|R|R| Message Type | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Originator Handle |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Remote Handle |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number |
| Sending Timestamp (seconds) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sending Timestamp (microseconds) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Receiving Timestamp (seconds) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Receiving Timestamp (microseconds) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TLVs |
. .
. .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1 SFC OAM message
o Version: version of SFC OAM message. This field is 8 bits long,
and current version is set to 0x01.
o Message Type indicate the type of SFC OAM message.
The SFC OAM message has the following types:
Value Meaning
----- -------
1 continuity check message
2 trace request message
3 trace reply message
o Originator Handle: The Originator Handle is filled in by the
packet original sender.
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o Remote Handle: The Remote Handle indicates the sink point. It
usually used in verify part of SFP. It should be filled with
0xffff when verify the whole SFP.
o Sequence Number: The Sequence Number is assigned by the sender of
the SFC request message and can be used to track the correct reply
message.
o The Sending Timestamp is the time-of-day (in seconds and
microseconds, according to the sender's clock) when the SFC OAM
request is sent.
o The Receiving Timestamp in an SFC OAM reply message is the time-
of-day (according to the receiver's clock) that the corresponding
request was received.
o TLVs (Type-Length-Value) have the following format:
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 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value |
. .
. .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2 SFC OAM TLVs
Types of SFC OAM TLV will be defined in the next revision; Length is
the length of the Value field in octets; and Value field is variable
depending on its Type (it is zero padded to align to a 4-octet
boundary).
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3. Theory of Operation
In order to describe SFC OAM in an abstract way, we reuse some
nomenclatures in MPLS WG. SFC OAM operates in the context of
Maintenance Entities (MEs) that define a relationship between two
points of a service function path to which maintenance and monitoring
operations apply. The two points that define a maintenance entity are
called Maintenance Entity Group End Points (MEPs).
An abstract reference model for an ME is illustrated in Figure 4
below:
+-+ +-+ +-+ +-+
|A|----|B|----|C|----|D|
+-+ +-+ +-+ +-+
Figure 3 SFC OAM Reference Model
In Figure 4, node A can be a classifier or an entry SFF, node D can
be an exit SFF, and node B and C can be any SFF or SF (with the
restriction that any two SFs cannot be directly connected in SFC
forwarding layer) on the SFC path.
In general, MEG End Points (MEPs) are the source and sink points of a
MEG for SFC OAM.
3.1. Continuity Check and Connectivity Verification of SFC
Proactive Continuity Check (CC) can be used to detect a loss of
continuity defect between two MEPs in a MEG.
Proactive Connectivity Verification (CV) can be used to detect an
unexpected connectivity defect between two MEGs or unexpected
connectivity within the MEG with an unexpected MEP.
BFD can also be used as a tool of proactive CC & CV in SFC, where BFD
Control packets must be sent along the same path as the monitored SFC
path.
3.1.1. MEP sending an SFC CC-CV packet
A source MEP can proactively sends CC-CV packets periodically to its
sink peer MEP. An SFC CC-CV packet is an SFC CC-CV message
encapsulated with an SFC Header. The SFC header is set as described
in [I-D. draft-ietf-sfc-nsh-00] and its flag O MUST be set to 1.
The SFC OAM message is set as follows:
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- The message type MUST be set to 1.
- The Sender's Handle is set by the original sender, and MUST be set
with the sender's identifier.
- The Sequence Number is set with a random value.
3.1.2. MEP terminating an SFC CC-CV packet
A sink MEP detects a loss of continuity defect when it fails to
receive proactive CC-V OAM packets from the source MEP for a
consecutive time.
When CC-V packets are received by a sink MEP, it is parsed. If any
mis-connectivity defect is detected, a warning should be raised and
fault management system should be notified of the detected defects.
3.2. SFC Route Tracing
According to the SFC architecture [I-D. draft-ietf-sfc-architecture-
09], SFC can be categorized into two abstraction layers, that is,
service function layer and SFC forwarding layer. In the service
function layer, a service function chain actually is a service
function graph, where a service function is connected to another
service function one by one in sequence. In the SFC forwarding layer,
service functions are further attached to SFF nodes thus form a more
detailed forwarding graph. As defects can be located on either
service functions or SFF nodes, it is critical to trace route both
service functions and SFF nodes to detect and isolate any defects for
SFC.
In order to trace route of a service function chain, different layers
of service function chain can be monitored:
o Service-function-layer, that is, only SF identifiers can be set as
the destination MEP in the trace route request and response
messages. The trace routing operation collects all the SFs'
identifiers along an SFC path. By comparing this SF list with the
pre-configured service function graph, an operator could determine
whether there is any fault in the SF connectivity and locate the
defect on an SF when there are any of them.
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o SFC-forwarding-layer, that is, both SF identifiers and SFF can be
set as the destination MEP in the trace route request and response
messages. The trace routing operation collects all the SFs'
identifiers and SFF identifiers along an SFC path. By comparing
this SF and SFF list with the pre-configured SFC forwarding graph,
an operator could determine whether there is any fault in the
forwarding layer and locate the defect on an SFF or an SF.
Furthermore, two different mechanisms may be used to trace route a
service function chain:
o TTL mechanism
Similar to the IP trace route, the detection node launches a number
of trace request messages in sequence to detect the fault in a
specific path, the TTL of request message is set successively to 1,
2, ..., and so on.
The trace route request will pass the SFs along the service function
graph, and each SF will decrease the TTL value by 1.
A trace route reply message will be generated and send back to the
launcher when the resulted TTL is equal to zero.
In this way, the launcher of trace routing can get the list of SFs
that the trace route request message passes by parsing all the trace
route reply messages, and isolate the fault location if there is any.
o record route mechanism
The detection node launches a single trace route request message, and
this message is transported over the specific SFC path.
When the trace route request message is received by an SF in the SFC
path, the SF adds its SF identifier to the end of an SF list carried
in the message. Moreover, a trace route reply message should be
generated and sent back to the launcher, and the new record route SF
list MUST be copied to the trace route reply message.
In this way, the launcher of trace routing can get the list of SFs
that the trace route request message passes by parsing all the trace
route reply messages, and isolate the fault location if there is any.
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3.2.1. MEP sending an SFC Trace Request
In general, MEG End Points (MEPs) are the source and sink points of a
MEG for SFC OAM. An MEP initiates a trace route request packet to
detect and track any fault in a Service Function Chain.
An SFC Trace route request packet is an SFC trace route request
message encapsulated with an SFC Header. The SFC header is set as
described in [I-D.draft-ietf-sfc-nsh-00] and flag O MUST be set to 1.
The SFC OAM message is further set as follows:
- The message type MUST be set to 2.
- The Sender's Handle MUST be set to the sender's identifier.
- The Receiver's Handle can be set to the exit SFF's identifier.
3.2.2. SFF processing an SFC Trace Route Request
When an SFF receives a trace route request packet with O flag being
set in SFC header, it firstly adds its identifier to the end of the
record route list in the trace request. It then performs service
forwarding function, and sends the new trace route request packet to
the next SF or next SFF.
Furthermore, the SFF sends a trace reply packet back to the source
MEP with a copy of the new record route SF list.
3.2.3. Service Function treating an SFC Trace Request
An SF can only be configured as an MIP in an MEG of SFC. When an SF
(being an MIP) receives a trace request packet with OAM flag being
set in SFC header from an SFF, it only sends it back to the SFF
transparently.
3.2.4. MEP receiving an SFC Trace Reply
An MEP should only process an SFC trace reply packet in response to
an SFC trace request that it has sent. Thus, upon receipt of an SFC
trace reply packet, an MEP should try to match the trace reply packet
with a trace request that it has previously sent, by checking the
corresponding path identifier and Sequence Number in the SFC OAM
packets. If no match is found, then the MEP MUST drop the trace reply
packet silently.
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Since each SFF in the SFC path will send a trace reply packet when
the trace request packet passes it, a source MEP will receive a
sequence of trace reply packets from SFFs (other than the MEP itself)
along the SFC path. Thus, the source MEP can get the full service
topology and SFC path if there is no defect in the SFC data plane,
and could detect and locate the data plane defects if there are any
of them.
4. Security Considerations
It will be considered in a future revision.
5. IANA Considerations
It will be considered in a future revision.
6. References
6.1. Normative References
6.2. Informative References
[I-D.ietf-sfc-problem-statement] P. Quinn, and T. Nadeau; Service
Function Chaining Problem Statement; August 2014; Work in
Progress
[I-D.ietf-sfc-architecture] J. Halpern, C. Pignataro; Service
Function Chaining (SFC) Architecture; September 2014; Work
in Progress
[I-D.aldrin-sfc-oam-framework] S. Aldrin; C. Pignataro; N. Akiya
Service Function Chaining Operations, Administration and
Maintenance Framework; July 2014; Work in Progress
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7. Acknowledgments
TBD
Authors' Addresses
Yuanlong Jiang
Huawei Technologies Co., Ltd.
Bantian, Longgang district
Shenzhen 518129, China
Email: jiangyuanlong@huawei.com
Weiping Xu
Huawei Technologies Co., Ltd.
Bantian, Longgang district
Shenzhen 518129, China
Email: xuweiping@huawei.com
Zhen Cao
Email: zhencao.ietf@gmail.com
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